#include <iostream>
#include <cstdio>

using namespace std;

double valoareAbsoluta(double d)
{
  return (d > 0) ? d : -d;
}

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    double aria = 0;
    scanf("%d", &n);
    double x1, x2, x3, y1, y2, y3;

    scanf("%lf", &x2);
    scanf("%lf", &y2);
    x1 = x2;
    y1 = y2;

    if(n <= 2)
      printf("0");
    else
    {
      for(int i = 1; i < n; ++i)
      {
        scanf("%lf", &x3);
        scanf("%lf", &y3);
        aria += x2 * y3 - x3 * y2;
        x2 = x3;
        y2 = y3;
      }
      aria += x3 * y1 - x1 * y3;
      printf("%.5lf", valoareAbsoluta(aria / 2));
    }


    return 0;
}