#include <bits/stdc++.h>

using namespace std;

const int NMax = 100005;

struct Point{
    long double x, y;
};

int n;
long double ans;
Point a[NMax];

long double CrossProduct(Point a, Point b) {
    return (a.x * b.y - a.y * b.x);
}
int main(){
    cin >> n;
    for(int i = 0; i < n; ++i){
        cin >> a[i].x >> a[i].y;
        if(i >= 1)
            ans += CrossProduct(a[i - 1], a[i]) / 2.0;
    }
    ans += CrossProduct(a[n - 1], a[0]) / 2.0;
    cout << fixed << setprecision(7) << ans << '\n';
    return 0;
}