#include <bits/stdc++.h>

using namespace std;

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

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

int n;
double S;

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);

    fout << S/2;

    return 0;

}