My code for 2085D is 313703210. I understood what the solution was, but this code must be working too. Does anybody have any ideas why isn't that works?

Today, I submitted a code on Codeforces, and it got a WA on test 2. When I looked up what is wrong with my code, 312665141 it says "wrong answer 5th numbers differ — expected: '3', found: '6' ", but when I try my code on compiler, it gave me actual value. So, how can I fix it?

In codeforces Div. 2 problems, I could only solve the first 2 problems in the contest, but out of contest I could write problem C too. How can I increase this value in contests?