/in/foo.cc: In function 'int main()':
/in/foo.cc:13:11: error: expected primary-expression before ';' token
  for (i = ; i <= n; i++)
           ^
/in/foo.cc:30:16: error: 'abe' was not declared in this scope
    a[i + 1] -= abe - a[i];
                ^~~
/in/foo.cc:30:16: note: suggested alternative: 'ave'
    a[i + 1] -= abe - a[i];
                ^~~
                ave