#include<stdio.h>

int main() {
    int a,b,c,d, maxim;
    scanf("%d", &a);
    scanf("%d", &b);
    scanf("%d", &c);
    scanf("%d", &d);
    maxim = -1;
    if(a*b + c*d > maxim) maxim = a*b + c *d;
    if(a*c + b*d > maxim) maxim = a*c + b *d;
    if(a*d + c*b > maxim) maxim = a*d + c *b;
    printf("%d", maxim);
    return 0;
}