#include<bits/stdc++.h>
using namespace std;
int main( )
{
int x,n;
double y,p,sum1,sum2;
cin>>x>>y>>n>>p;
sum2=p*n/10.0;
if (p>=x)
{
sum1=p-y;
if (sum1<=sum2)
cout<<fixed<<setprecision(2)<<sum1;
else
cout<<fixed<<setprecision(2)<<sum2;

}
else
cout<<fixed<<setprecision(2)<<sum2;

return 0;

}