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

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

double len(double x1,double x2,double y1,double y2){
  return sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
}
  

int main(){
  double x[21],y[21];
  double s=0.0;
  int n=0;
  while(true){
    cin >>x[n]>>y[n];
    if(!cin) break;
    n++;
  }
  for(int i=0;i<n-2;i++){
    double a,b,c;
    a=len(x[i],x[i+1],y[i],y[i+1]);
    b=len(x[i+1],x[i+2],y[i+1],y[i+2]);
    c=len(x[i+2],x[i],y[i+2],y[i]);
    //cout<<a<<b<<c<<endl;
    s+=area(a,b,c);
    //cout<<s<<endl;
  }
  cout<<s<<endl;
}