View Compilation Error

0_0_24675435_19940.c: In function 'main':
0_0_24675435_19940.c:44:2: error: expected expression before '/' token
  //freopen("input.txt", "r", stdin);
  ^
0_0_24675435_19940.c:51:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for (int i = 1; i <= N; ++i)
   ^
0_0_24675435_19940.c:51:3: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_24675435_19940.c:60:12: error: redefinition of 'i'
   for (int i = 1; i <= N; ++i)
            ^
0_0_24675435_19940.c:51:12: note: previous definition of 'i' was here
   for (int i = 1; i <= N; ++i)
            ^
0_0_24675435_19940.c:60:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for (int i = 1; i <= N; ++i)
   ^