#include<iostream>
#include<fstream>
#include<algorithm>
#include<cstring>
#include<vector>
#include<string>
#include<queue>
#define in cin
#define out cout
#define abs(x) ((x>0)?(x):(-(x)))
#define max(a, b) (a > b ? a : b)
#define min(a, b) (a < b ? a : b)
#define FOR(i, a, b) for(int i = a; i <= b; ++i)
#define DOWNFOR(i, a, b) for(int i = a; i >= b; --i)
#define FOREACH(i, t) for (typeof(t.begin()) i = t.begin(); i != t.end(); ++i)
#define ll long long
using namespace std;
int main(){
    #ifndef ONLINE_JUDGE
    ifstream in("test.in");
    ofstream out("test.out");
    #endif
    int a,b,c,d;
    in>>a>>b>>c>>d;
    int m=0;
    m=max(m,a*b+c*d);
    m=max(m,a*c+b*d);
    m=max(m,a*d+b*c);
    out<<m<<'\n';
    return 0;
}