题意：给出三角形的三条中线长度求面积。

#include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;

int main()
{
    double n,m,p;
    while(cin>>n>>m>>p)
    {
        if((m+n+p)*(m+p-n)*(m+n-p)*(n+p-m)<=0||m<=0||n<=0||p<=0||m+p<=n||n+p<=m||m+n<=p) puts("-1.000");
        else printf("%.3lf\n",sqrt((m+n+p)*(m+p-n)*(m+n-p)*(n+p-m))/3.0);
    }
    return 0;
}