#include <iostream>
#include <fstream>
#include <string.h>
#include <algorithm>
#include <fstream>
#include <bitset>
#include <map>
#include <vector>
#include <queue>
#include <stack>
#include <utility>
#include <string>
#include <cmath>
#include <iomanip>
#include <cstring>

#define vint vector<int>::iterator
#define vintp vector<pair<int,int> >::iterator
#define inf 1000000
#define ll long long
#define maxn 100010
#define mod 1000000007

using namespace std;

ifstream fin ("A.in");
ofstream fout("A.out");

int a,b,c,d,maxv;

int main ()
{
    cin>>a>>b>>c>>d;

    maxv = max (maxv,a*b+c*d);
    maxv = max (maxv,a*c+b*d);
    maxv = max (maxv,a*d+b*c);
    cout<<maxv;
}