#include<bits/stdc++.h>
using namespace std;

int main()
{
	double x,y,n,p;
	cin>>x>>y>>n>>p;
	double a=0,b=0;
	b=p/10*n;
	if(p>=x)
	{
		a=p-y;
	}
	else
	{
		printf("%.2lf",b);
		return 0;
	} 
	if(a<=b)
	{
		printf("%.2lf",a);
	}
	else
	{
		printf("%.2lf",b);
	}
	
	return 0;
}

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;
}