#include <bits/stdc++.h>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct
{
    double x, y;
} v[100001];
double arie;
int n,i;
int main()
{
    fin>>n;
    for(i=1; i<=n; ++i)
        fin>>v[i].x>>v[i].y;
    n++;
    v[n].x=v[1].x;
    v[n].y=v[1].y;
    for(i=1; i<n; ++i)
        arie += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);
    fout<<arie / 2.0;
    return 0;
}