#include <complex>
#include <cmath>
#include <iostream>
using namespace std;
typedef complex<double> xy_t;
xy_t P[110], a, b, c;

double cross_product(xy_t a,xy_t b) {
  return (conj(a)*b).imag()/2.0;
}

int main () {
  int N=0; //??????????????????????????°???N??¨??????
  double x, y;
  while (cin>x && cin>y) {
    P[N++] = xy_t(x,y);
  }
  //??¢????¨????
  double sum = 0.0;
  for (int i=0; i+2<N; ++i) {
    xy_t b=P[i+1], c=P[i+2];
    sum += cross_product(b, c);
  }
  cout << abs(sum) <<endl;
}