1) Maybe, you are reading long long variable with this command :
  scanf("%d",&arr[i]);
  It is not right. You should do, for example, it with next command
  scanf("%I64d",&arr[i]);
2) Maybe, you were confused by statement. You have to choose such two points so line that passing through this points has maximal absolute value of inclination, not just inclination.
3) Maybe, you are doing something like this :
    for ( int i = 1 ; i <= n ; ++i ) {
      long long d = abs2(arr[i+1] - arr[i]);

       if ( d > x ) {
                 x = d; m1 = i;
       }
   }
There is a mistake in first line. Right way is
   for ( int i = 1 ; i < n ; ++i ) {

It was my biggest mistake, because the problem itself is so simple.

Thannnnnnnnnnnnnnnnnk youuuuuuuuuuuuuu verrrrrrrrrrrrry mucccccccccccccch!

Thannnnnnnnnnnnnnnnnk youuuuuuuuuuuuuu verrrrrrrrrrrrry mucccccccccccccch!

Thanks a lot! The definition of inclination is ambiguous in the problem statement.

Thanks a lot! I have ignored "absolute value" too.

Thannnnnnnnnnnnnnnnnk youuuuuuuuuuuuuu verrrrrrrrrrrrry mucccccccccccccch!

Thannnnnnnnnnnnnnnnnk youuuuuuuuuuuuuu verrrrrrrrrrrrry mucccccccccccccch!