C

#include<bits/stdc++.h>
using namespace std;
int a[10];
double x,y,z;
int main()
{
    int n;
    cin >> n;
    for (int i = 1;i <= n;i ++) cin >> a[i];
    double ma = 0;
    bool flag = false;
    for (int i = 1;i <= n-2;i ++)
    {
        x = 0;
        for (int j = 1;j <= i;j ++) x += a[j];
        for (int k = i + 1;k <= n-1;k ++)
        {
            y = 0;
            for (int q = i + 1;q <= k;q ++) y += a[q];
            z = 0;
            for (int e = k + 1;e <= n;e ++) z += a[e];
            if (x+y<=z||x+z<=y||y+z<=x) 
            {
                continue;
            }
            else
            {
                double p = (x+y+z)/2;
                double S = sqrt(p*(p-x)*(p-y)*(p-z));
                if (S >= ma) ma = S;
                flag = true;
            }
        }
    }
    
    if (flag) printf("%.1lf",ma);
    else cout << -1;
    
    return 0;
}