/in/foo.cc: In function 'int main()':
/in/foo.cc:25:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int p=0;p<a[i].size();p++) {
                 ~^~~~~~~~~~~~
/in/foo.cc:26:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int q=0;q<a[j].size();q++) {
                  ~^~~~~~~~~~~~