#include<bits/stdc++.h>
using namespace std;
int main() {
    int x, y, n, p;
    cin >> x >> y >> n >> p;
double a, b; 
    if (p >= x) {
        a = p - y;
    } else {
        a = p; 
    }
    b = p * n / 10.0; 
    cout << fixed << setprecision(2) << min(a, b);
return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main()
{
    double x,y,n,p;
    cin>>x>>y>>n>>p;
    if(p-double(int(p)/int(x)*y)<p*(n/10))printf("%.2f",p-double(int(p)/int(x)*y));
    else printf("%.2f",p*(n/10));
}

C++ :

#include <algorithm>
#include <cstdio>
using namespace std;
	int x, y, n, p;
	double a1, a2;
	int main() {
	scanf("%d%d%d%d", &x, &y, &n, &p);
	a1 = p;
	if (a1 >= x) a1 -= y;
	a2 = p * 0.1 * n;
	printf("%.2lf\n", min(a1, a2));
	return 0;
}