#include<iostream>
using namespace std;
int main()
{
    int x,y,w,n;
    while(cin>>x>>y>>w>>n)
        cout<<(n-1-(x/w+1)*((x/w+n)/(x/w+1)-1))*w+(x+y)*((x/w+n)/(x/w+1)-1)<<endl;
    return 0;
}