/in/foo.cc: In function 'int main()':
/in/foo.cc:10:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0; i<s.length(); i++){
               ~^~~~~~~~~~~
/in/foo.cc:15:7: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
   sum += (a[i]-'0')*(i+1);
   ~~~~^~~~~~~~~~~~~~~~~~~