The difficulty index of a problem is defined as:

number of solves for p1/p4

1.3 * number of solves for p2/p5

1.6 * number of solves for p3/p6

solve is defined as score of at least 5

Roughly, the lower the difficulty index is, the harder the problem is. This market resolves YES if and only if there exists a pair of problems (which may or may not be from the same day) such that the problem that was placed strictly later within the day that the problem was from has a strictly higher difficulty index than the one that was placed earlier.

The 1.3 and 1.6 multipliers are there to account for artificial difficulty on later problems due to less people attempting them seriously. If 2 and 3 have the same number of solves, this actually indicates that 2 was harder than 3.

Again, this market will temporarily close before USAMO due to avoid bets when discussion is forbidden

If you have a better way of comparing difficulty, suggest in comments and I might implement it