#include <bits/stdc++.h>
#define x first
#define y second
/// TONI BO$$ was here
/// #MLC

using namespace std;

pair <long double,long double> v[100001];
int main()
{
    int n,i,j;
    ios::sync_with_stdio(false);
    ifstream in("aria.in");
	ofstream out("aria.out");
    in>>n;
    long double s=0;
    for(i=0; i<n; i++)
        in>>v[i].x>>v[i].y;
    for(i=0; i<n; i++)
    {
        j=(i+1)%n;
        s+=(v[i].x*v[j].y-v[i].y*v[j].x);
    }
    out<<fixed<<setprecision(8)<<s/2.0;

    return 0;
}