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

double Z(double a, double b, double c){
    return (a+b+c)/2.0;
}
double s(double a, double b, double c){
    double S, z;
    z = Z(a, b, c);
    S = z*(z-a)*(z-b)*(z-c);
    return sqrt(S);
}

int main(void){
    int n;
    double x[100], y[100], sum=0;
    double a, b, xx, yy, tt;
    for(int i=1; cin>>a>>b; i++){
        x[i] = a;
        y[i] = b;
        n = i;
    }
    for(int i=2; i<=n; i++){
        xx = sqrt((x[i]-x[i-1])*(x[i]-x[i-1])+(y[i]-y[i-1])*(y[i]-y[i-1]));
        yy = sqrt(x[i]*x[i]+y[i]*y[i]);
        tt = sqrt(x[i-1]*x[i-1]+y[i-1]*y[i-1]);
        sum+=s(xx, yy, tt);
    }
    xx = sqrt((x[n]-x[1])*(x[n]-x[1])+(y[n]-y[1])*(y[n]-y[1]));
    yy = sqrt(x[n]*x[n]+y[n]*y[n]);
    tt = sqrt(x[1]*x[1]+y[1]*y[1]);
    
    sum+=s(xx, yy, tt);
    cout << sum << endl;
    return 0;
}

