/in/foo.cc: In function 'int main()':
/in/foo.cc:16:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int i = 0; i < isbn.size() - 1; i++)
                    ~~^~~~~~~~~~~~~~~~~
/in/foo.cc:26:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < isbn.size() - 1; i++)
                     ~~^~~~~~~~~~~~~~~~~
/in/foo.cc:35:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < isbn.size() - 1; i++)
                     ~~^~~~~~~~~~~~~~~~~