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

using namespace std;

pair <double,double> v[100001];
int main()
{
    int n,i,j;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    double s=0;
    for(i=0; i<n; i++)
        scanf("%lf%lf",&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);
    }
    printf("%lf",s/2.0);

    return 0;
}