#include <iostream>
#include <cstdio>

using namespace std;

double x,y,A_tot;
int n;

struct {
    double first,second;
}v[100025];


void citire ()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    scanf("%d",&n);

    for (int i=1; i<=n; ++i)
    {
        scanf("%lf%lf",&x,&y);
        v[i].first=x;
        v[i].second=y;
    }


}

int main()
{
    citire();

    v[0].first=v[n].first;
    v[0].second=v[n].second;

    for (int i=1;i<=n;++i)
        A_tot+=(v[i-1].first-v[i].first)*(v[i].second+v[i-1].second);
    A_tot/=2;

    printf("%lf",A_tot);

    return 0;
}