#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int n;
double s;

struct elem {
    double x, y;
} a[100005];

int main()
{
    fin >> n;
    for(int i=1; i<=n; i++)
        fin >> a[i].x >> a[i].y;
    a[n+1]=a[1];
    for(int i=1; i<=n; i++)
        s+=(a[i].x*a[i+1].y - a[i+1].x*a[i].y);
    s/=2;
    fout << setprecision(6) << fixed << s;
    return 0;
}