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;
}