#include <cstdio>
#include <algorithm>

#define NMAX 100005
#define X first
#define Y second

using namespace std;

int n;
pair<double,double> p[NMAX];

void read()
{
    scanf("%d",&n);
    for(int i=1;i<=n;++i)
        scanf("%lf%lf",&p[i].X,&p[i].Y);
}

void solve()
{
    double aria=0;
    for(int i=1;i<n;++i)
        aria+=p[i].X*p[i+1].Y-p[i].Y*p[i+1].X;
    aria+=p[n].X*p[1].Y-p[n].Y*p[1].X;
    printf("%.5lf",aria/2);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    read();
    solve();
    return 0;
}