#include <bits/stdc++.h>
#define NMAX 100000

using namespace std;

struct COORD {
  double x, y;
}v[NMAX];

int main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    int n, i;
    fin>>n;
    for( i = 0; i < n; i ++ )
      fin>>v[i].x>>v[i].y;
    long double S = 0;
    for( i = 0; i <= n; i ++ )
      S += v[i].x * v[(i+1)%n].y - v[(i+1)%n].x * v[i].y;
    S = S / (double)2;
    fout<<S;
    return 0;
}