#include <cstdio>
#include <algorithm>
using namespace std;

int main() {
    int a, b, c, d;
    
    scanf("%d %d %d %d", &a, &b, &c, &d);
    
    int s1 = a*b+c*d;
    int s2 = a*c+b*d;
    int s3 = a*d+b*c;
    
    int m = max( max(s1, s2), s3);
    printf("%d\n", m);
    
    return 0;
}