#include <iostream>
#include <string.h>
 
using namespace std;
int main() {
    long M, F, B, N, K, O, R;
    cin >> M >> F >> B >> N;
    O = M % (N * (B + F));
    K = M - O;
    R = O - O % F;
    K += min(R, F*(N-1));
    cout << (M-K) << endl;
    return 0;
}