/in/foo.cc: In function 'int main()':
/in/foo.cc:5:16: warning: unused variable 'i' [-Wunused-variable]
  long long n,k,i,j;
                ^
/in/foo.cc:5:18: warning: unused variable 'j' [-Wunused-variable]
  long long n,k,i,j;
                  ^
/in/foo.cc:10:4: warning: 's' is used uninitialized in this function [-Wuninitialized]
   s=s+1.0/n;
   ~^~~~~~~~