foo.cc: In function ‘int main()’:
foo.cc:5:15: warning: unused variable ‘i’ [-Wunused-variable]
    5 |         int n,i;
      |               ^
foo.cc:11:18: warning: ‘s’ may be used uninitialized [-Wmaybe-uninitialized]
   11 |                 s+=a;
      |                 ~^~~
foo.cc:6:18: note: ‘s’ was declared here
    6 |         double a,s;
      |                  ^