#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
using namespace std;
int x, y, w, n;
int a[110], al;
int main (){
    int i, j, k;
    while ( scanf ( "%d%d%d%d", &x, &y, &w, &n ) != EOF ){
        al = 0;
        for ( i = 0; i <= x; i += w ){
            a[++al] = i;
        }
        int k1 = n/al, k2 = n%al;
        if ( k2 == 0 ){ k2 = al; k1 --; }
        printf ( "%d\n", (x+y)*k1+a[k2] );
    }
    return 0;
}