#include<fstream>
using namespace std;
ifstream in ("aria.in");
ofstream out ("aria.out");
long long aria,x1,x2,x3,y1,y2,y3,n;
int abs (int x) {
    if (x < 0) {
        return -x;
    }
    return x;
}
pair<int,int> v[100001];
int main (void) {
    in >> n;
    for (int i = 1; i <= n; i ++) {
        in >> v[i].first >> v[i].second;
    }
    x1 = 0;
    y1 = 0;
    v[n+1] = v[1];
    for (int i = 1; i <= n; i ++) {
        x2 = v[i].first;
        y2 = v[i].second;
        x3 = v[i+1].first;
        y3 = v[i+1].second;
        aria += ((x2-x1)*(y3-y1) - (x3-x1)*(y2-y1)) / 2;
    }
    out << abs(aria);

    return 0;
}