#include <bits/stdc++.h>

using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
int n,i,j;
struct hatz
{
    long double x,y;
};
hatz p[100005];
long double x;
int main()
{
    fin>>n;
    x=0;
    for (i=1; i<=n; i++)
    {
        fin>>p[i].x>>p[i].y;
    }
    p[n+1].x=p[1].x;
    p[n+1].y=p[1].y;
    for (i=1; i<=n; i++)
    {
        x=x+(p[i].x*p[i+1].y-p[i+1].x*p[i].y);
    }
    fout<<x/2;
    fout.close();
    return 0;
}