#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

int main(){
    long double xa,ya,xb,yb,xc,yc;
    cin>>xa>>ya>>xb>>yb>>xc>>yc;

    cout<<fixed<<setprecision(9)<<
        sqrt((xa-xb)*(xa-xb)+(ya-yb)*(ya-yb)) *
        sqrt((xa-xc)*(xa-xc)+(ya-yc)*(ya-yc)) /2 <<'\n';
}