/in/foo.cc: In member function 'bignum bignum::operator*(const bignum&)':
/in/foo.cc:31:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int s = 0; s <= d1 + d2 - 2; s++){
                        ~~^~~~~~~~~~~~~~
/in/foo.cc:33:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(int k = 0; k <= d1 - 1; k++){
                            ~~^~~~~~~~~
/in/foo.cc:34:40: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
                 if(s - k >= 0 && s - k <= d2 - 1) ret.a[s] += a[k] * b.a[s - k];
                                  ~~~~~~^~~~~~~~~
/in/foo.cc:38:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i = 0; i < d1 + d2 - 2; i++){
                        ~~^~~~~~~~~~~~~