/in/foo.cc: In function 'int main()':
/in/foo.cc:11:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<x.size();i+=2){
               ~^~~~~~~~~
/in/foo.cc:26:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<x.size();i++)sum+=x[i]-'0';
               ~^~~~~~~~~