foo.cc: In function 'int main()':
foo.cc:15:13: error: 'tt' was not declared in this scope
   s+=k*(ss==tt);
             ^~
foo.cc:15:13: note: suggested alternative: 't'
   s+=k*(ss==tt);
             ^~
             t

自豪的采用 HydroJudge 进行评测(github.com/hydro-dev/HydroJudge)