#include<bits/stdc++.h>
#define int long long
#define double long double
#define lowbit(x) x&(-x)
#define fastios ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define endl '\n'
#define el puts("")
#define udm unordered_map<int,int>
#define set_0(a) memset(&a,0,sizeof(a))
#define set_unint(a) memset(&a,-1,sizeof(a))
using namespace std;
//define_var
const int M=2e5+9,inf=1e18;
//define_var
//function_begin
int read()
{
	int x=0,f=1;
	char c=getchar();
	while(c<'0'||c>'9'){if(c=='-') f=-1;c=getchar();}
	while(c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
	return x*f;
}
void write(int x)
{
	if(x<0) putchar('-'),x=-x;
	if(x>9) write(x/10);
	putchar(x%10+'0');
}
//function_end
//solve_begin
void solve()
{
	int a[4];
	for(int i=0;i<4;i++)
		cin>>a[i];
	int res=-1e18;
	for(int i=0;i<4;i++)
	{
		for(int j=0;j<4;j++)
		{
			if(i==j) continue;
			for(int s=0;s<4;s++)
			{
				if(i==s||j==s) continue;
				for(int t=0;t<4;t++)
				{
					if(i==t||j==t||s==t) continue;
					int ans=min(a[i],a[j])*min(a[s],a[t]);
					res=max(res,ans);
				}
			}
		}
	}
	cout<<res<<endl;
}
//solve_end
//main_begin
signed main()
{
	fastios
	int T=1;
//	cin>>T;
	while(T--)
		solve();
}
//main_end