#include <cstdio>
#include <iostream>
#define nmax 100005
#define x first
#define y second

using namespace std;

pair<double,double> p[nmax];
double arie;
int n;

void citire()
{
    double xx,yy;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%lf%lf",&xx,&yy);
        p[i]=make_pair(xx,yy);
    }
}

void aria()
{
    for(int i=1;i<n;i++)
        arie=arie+p[i].x*p[i+1].y-p[i+1].x*p[i].y;
    arie+=p[n].x*p[1].y-p[1].x*p[n].y;
}

double mod(double x)
{
    if(x<0)
        return -x;
    return x;
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    citire();
    aria();
    printf("%.5lf\n",1.0/2.0*arie);
    return 0;
}