#include<iostream>
#include<complex>
#include<cstdio>
#include<cmath>

typedef std::complex<double> point;

double CalS(double a,double b,double c){
    double z=(a+b+c)/2;
     return sqrt(z*(z-a)*(z-b)*(z-c));
}

int main(void){
    point p[20];
    int n=0;
    double x,y;
    while(scanf("%lf%lf",&x,&y)!=EOF)
        p[n++]=point(x,y);
    double sum=0;
    for(int i=1;i<n-1;i++){
        sum+=CalS(std::abs(p[0]-p[i]),std::abs(p[0]-p[i+1]),std::abs(p[i]-p[i+1]));
    }
    printf("%lf\n",sum);
    return 0;
}