#include <stdio.h>

int n;
int v[10];

inline int max (int a, int b) {return a > b ? a : b;}

int main ()
{
//	freopen ("mind.in", "r", stdin);
//	freopen ("mind.out", "w", stdout);
	
	scanf ("%d %d %d %d", &v[1], &v[2], &v[3], &v[4]);
	
	int i, j, k, l, sol = 0;
	for (i = 1; i <= 4; i ++)
		for (j = 1; j <= 4; j ++)
			for (k = 1; k <= 4; k ++)
				for (l = 1; l <= 4; l ++)
					if (i != j && i != k && i != l && j != k && j !=l && k != l)
						sol = max (sol, v[i] * v[j] + v[k] * v[l]);
	printf ("%d\n", sol);
	return 0;
}