Well, I can prove that the expn is not divisible by 1991;
This can be done like this
1992-(1) is div by 1991
therefore 1992≡1(mod1991)
=> 19921991 ≡1(mod1991)
=>199219911990≡1(mod1991)...1
similarly 1990-(-1) is div by 1991
proceeding in the same way , we get
199019911992≡1(mod 1991)....2
Adding 1 and 2
19901991 1992 +199219911990≡2(mod1991)
If this method is correct, how can we conclude that 0 is the highest possible value?