include<bits/stdc++.h>

using namespace std; int main(){

int x,y,n,p;
cin>>x>>y>>n>>p;
double c1;
if(p>=x){
    c1=p-y;
} 
else{
    c1=p;
}
double c2=p*n/10.0;
double min;
if(c1<c2){
    min=c1;
} 
else{
    min=c2;
}
printf("%.2f",min);
return 0;

}