#include <bits/stdc++.h>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
struct point { int x,y; } a[100005];

double aria ()
{
    double s =0;
    for(int i=1; i<=n; i++)
        s += ( a[i].x*a[i+1].y - a[i].y*a[i+1].x);
    s /= 2;
    return s;
}
int main()
{
    ios::sync_with_stdio(0);
    in >> n;
    for(int i=1; i<=n; i++)
        in >> a[i].x >> a[i].y ;
    a[n+1] = a[1];
    double A = aria();
    out << setprecision(5) << A;

    return 0;
}