Main.cc:13:10: error: ‘::main’ must return ‘int’ int main() { ^ Main.cc: In function ‘int main()’: Main.cc:22:10: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%lld", &_); ~~~~~^~~~~~~~~~~~ Main.cc:24:14: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%lld%lld%lld", &n, &m, &K); ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~