#include<bits/stdc++.h>
using namespace std;
int main()
{
	long long a,k,r,c,s1=0,s2=0,s3=0;
	cin>>a>>k>>r>>c;
	s1=a*k*r;
	s2=c*k;
	s3=s1-s2;
	cout<<s3;
    return 0;
}

哪错了?