求调

#include<bits/stdc++.h>
using namespace std;
long double x, y, n, p, ans, sum;
int main(){
	cin >> x >> y >> n >> p;
	if(p >= x){
		ans = p * 1.0 - y;
	}
	sum = p * 1.0 * n / 10;
	cout << fixed << setprecision(2) << min(ans, sum);
	return 0;
}