警示后人：double p=(a+b+c)/2.0;

#include<bits/stdc++.h>
#define int long long
using namespace std;
double a,b,c,d,e,f,g;
double area(int a,int b,int c){
    double p=(a+b+c)/2.0;
    return sqrt(p*(p-a)*(p-b)*(p-c));
}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    cin>>a>>b>>c>>d>>e>>f>>g;
    cout<<fixed<<setprecision(2)<<area(a,b,c)+area(c,e,d)+area(e,g,f);
    return 0;
}