#include <bits/stdc++.h>

using namespace std;

ifstream fein ("aria.in");
ofstream fout ("aria.out");

int n;
long double S;

struct ceva{
    long double x;
    long double y;
}v[100010];

double arie (long double x1, long double y1, long double x2, long double y2){
    return ((x1 * y2 - x2 * y1) / 2);
}

int main()
{
    fein >> n;

    for (int i = 1; i <= n; ++i){
        fein >> v[i].x >> v[i].y;
    }

    v[n + 1] =  v[1];
    for (int i = 2; i <= n + 1; ++i){
        S += arie(v[i - 1].x, v[i - 1].y, v[i].x, v[i].y);
    }

    fout << setprecision(6) << fixed << S;
    return 0;
}