/in/foo.cc: In function 'int main()':
/in/foo.cc:7:17: error: 'n3' was not declared in this scope
  for(int i=1;i<=n3;i++){
                 ^~
/in/foo.cc:7:17: note: suggested alternative: 'n'
  for(int i=1;i<=n3;i++){
                 ^~
                 n
/in/foo.cc:4:8: warning: unused variable 'i' [-Wunused-variable]
  int n,i,s;
        ^