#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int H, W;
	cin >> H >> W;
	int A, B;
	cin >> A >> B;

	int X = H / A;
	int Y = W / B;
	cout << H * W - A * B*X*Y << endl;
}
