#include<bits/stdc++.h>
#define int long long
using namespace std;
auto IN = freopen("mmatrix.in", "r", stdin);
auto OUT = freopen("mmatrix.out", "w", stdout);
auto mread = [](){int x;scanf("%lld", &x);return x;};
int n = mread(), m = mread(), k1 = mread(), k2 = mread(), ans = LONG_LONG_MAX;
signed main(){
    if(n == 2 || m == 2){
        printf("%lld", (n - 1) * k1 + (m - 1) * k2);
        return 0;
    }
    if(n % 2 == 0){
        ans = min(ans, k1 * (n / 2));
    }
    if(m % 2 == 0){
        ans = min(ans, k2 * (m / 2));
    }
    printf("%lld", ans);
    return 0;
}