foo.cc: In function 'int main()':
foo.cc:24:6: error: 'else' without a previous 'if'
      else if(a=n-i-2)
      ^~~~
foo.cc:24:15: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
      else if(a=n-i-2)
              ~^~~~~~

[Hydro](https://hydro.ac)提供评测服务