#include <stdio.h>


using namespace std;

int main() {
    int v[5];
    for (int i = 1; i <= 4; ++i)
        scanf ("%d", &v[i]);
    int rez = v[1]*v[2] + v[3]*v[4];
    int rez2 = v[1]*v[3] + v[2]*v[4];
    int rez3 = v[1]*v[4] + v[2]*v[3];
    if (rez < rez2) rez = rez2;
    if (rez < rez3) rez = rez3;
    printf ("%d", rez);
    return 0;
}