#include <iostream>
using namespace std;
int main() {
// your code goes here
float a,c,b,d,p;
int x,y,n;
cin>>x;
cin>>y;
cin>>n;
cin>>p;
if(p<x) a=p;
else a=p-y;
b=p/10;
c=b*n;
d=min(a,c);
printf("%.2f",d);
return 0;
}
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