#include <bits/stdc++.h>
using namespace std;

int n;
pair <double, double> p[100010];
double ans;
#define x first
#define y second

int main(){
    ifstream cin ("aria.in");
    ofstream cout ("aria.out");
    cin >> n;
    for (int i=1; i<=n; i++) cin >> p[i].x >> p[i].y;
    p[0] = p[n];
    for (int i=1; i<=n; i++) ans += (p[i-1].x * p[i].y - p[i].x * p[i-1].y);
    cout << fixed << setprecision(5) << ans/2;
    return 0;
}