#include <iostream>
#include <cstdio>
#define NMAX 100010

using namespace std;

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

double x[NMAX], y[NMAX];

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int n;
    double aria = 0;
    scanf("%d", &n);
    for(int i = 0; i < n; ++i)
      scanf("%lf %lf", &x[i], &y[i]);
    x[n] = x[0];
    y[n] = y[0];
    for(int i = 0; i < n; ++i)
      aria += x[i] * y[i + 1] - x[i + 1] * y[i];
    printf("%.5lf", valoareAbsoluta(aria / 2));

    return 0;
}