#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
typedef long long ll;
using namespace std;

int main() {
	int a, b, c, d, e;cin>>a>>b>>c>>d>>e;
	int h = -1e8;
	for(int i = c; i>=0; i--){
		h = max(h, a+i*d-b-i*e);
	}
	cout << h << endl;
	return 0;
}