#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
using namespace std;
typedef long long ll;

int main()
{
    int n, m, x, y;
    cin >> n >> m >> x >> y;

    int p1 = 0, p2 = 0;
    for(int i = 0; i <= x; i++) {
        for(int j = 0; j <= x; j++) {
            if(i + j <= y) {
                p1 = i;
                p2 = j;
            }
        }
    }
    int sum1 = 0;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            if(i % 2 == 0 and j % 2 == 0) {
                sum1 += p1;
            }
            else if(i % 2 == 0 and j % 2 == 1) {
                sum1 += p2;
            }
            else if(i % 2 == 1 and j % 2 == 0) {
                sum1 += p2;
            }
            else {
                sum1 += p1;
            }
        }
    }
    int sum2 = 0;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            if(i % 2 == 0 and j % 2 == 0) {
                sum2 += p2;
            }
            else if(i % 2 == 0 and j % 2 == 1) {
                sum2 += p1;
            }
            else if(i % 2 == 1 and j % 2 == 0) {
                sum2 += p1;
            }
            else {
                sum2 += p2;
            }
        }
    }
    cout << max(sum1, sum2) << endl;
    return 0;
}
// 2222222222
// 22222222222