/in/foo.cc: In function 'int main()':
/in/foo.cc:44:7: error: 'ans' was not declared in this scope
    if(ans<0) 
       ^~~
/in/foo.cc:43:5: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
   if(f[5000-i]!=-1)
     ^
/in/foo.cc:49:7: error: 'ans' was not declared in this scope
    if(ans<0) 
       ^~~
/in/foo.cc:48:5: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
   if(f[5000+i]!=-1)
     ^