One that avoids a nuclear war, obviously, but it's equally obvious that neither party is going to abandon its nuclear weapons.
The United States, as the first country to build nuclear bombs and the only country ever to use them, sees having thousands of them as its birthright and would never consider giving them up.
North Korea's regime has only a few, but sees them as the only real guarantee of its survival.
But that can't be entirely true, because North Korea had already survived for 57 years before it tested its first nuclear explosive device in 2006.
It was another dozen years before it built a very small but theoretically effective force of nuclear-tipped ballistic missiles that could, on a good day and with a tailwind, reach the United States.
Only now has Pyongyang achieved nuclear deterrence against the US. What protected it before that?
What served North Korea as deterrence until 2017 was a very big army (twice the size of South Korea's army PLUS the American troops stationed in South Korea), and the ability to destroy Seoul within a day or two using only conventional artillery and rockets.
Seoul's northern suburbs are only 50km from the North Korean border, well within the range of many thousands of North Korean artillery pieces, and the metropolitan area is home to half of South Korea's 50 million people.
As the capital, it also contains almost all the government ministries and military headquarters. Not even US nuclear weapons could save it from destruction in a North-South war.
So forget about both sides' nuclear weapons and concentrate on the conventional balance.
South Korea has twice North Korea's population but only half as many soldiers on active service, because Seoul would rather save money and rely on (American-supplied) nuclear deterrence.
Now that North Korea has nukes of its own, it too can afford to shrink its army by at least half. In fact, it can't really afford not to.
Kim Jong Un would gain a lot if the summit actually happens. Just by sitting down with a US president as an equal he would win the kind of international legitimacy that always eluded his father and grandfather.
If he also got an American promise not to try to overthrow him and a suspension of US economic sanctions, his success would be complete. But what could he offer the US and South Korea in return?
Kim has already unilaterally suspended both nuclear weapons testing and further ballistic missile flight tests in order to attract Trump to the table, but he must come up with some other concessions to get the rest of what he wants.
How about a deal that commits him to reduce North Korea's army to the same size as South Korea's, and an agreement by both sides to move their artillery at least 50km back from the inter-Korean border?
That sort of deal would save Kim a lot of money without exposing him to any serious risk: it's his secret police, not the army, that keeps his population in line.
South Korea would still have no credible ability to attack the North, and Kim's own ability to threaten Seoul with a "sea of fire" would evaporate because he would first have to move his artillery back to the border area along roads totally exposed to US and South Korean air power.
This is what successful diplomatic deals actually look like. They are often asymmetric in some details, but they are more or less balanced overall and they give both parties what they really need.
What Trump needs is a diplomatic triumph that feeds his ego and maybe gets him the Nobel Peace Prize, while giving him a plausible excuse not to insist on the unattainable goal of eliminating North Korea's rudimentary nukes and ICBMs.
Kim can afford to give him concessions on other military issues, because even a 10 per cent chance that one North Korean ICBM could deliver one nuclear weapon on an American city is deterrent enough to preclude any US attack on North Korea.
In return, North Korea gets an end to sanctions and huge savings on its bloated military spending.
No promises, but this actually could happen.
And if Trump and Kim did get the Nobel Peace Prize for it, so what? It's meant as a reward for saints, but it works just as well as bait for scoundrels.
■Gwynne Dyer's new book is "Growing Pains: The Future of Democracy (and Work)".
