/in/foo.cc: In function 'int main()':
/in/foo.cc:14:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++){n*=10;n+=a[i]-'0';}
              ~^~~~~~~~~
/in/foo.cc:14:31: warning: 'n' may be used uninitialized in this function [-Wmaybe-uninitialized]
  for(int i=0;i<a.size();i++){n*=10;n+=a[i]-'0';}
                              ~^~~~