#include <bits/stdc++.h>

using namespace std;
ifstream f("arie.in");
ofstream g("arie.out");
struct punct
{
    long double x,y;
}a[100005];
int n;

void citire()
{
    f>>n;
    for(int i=1;i<=n;++i)
        f>>a[i].x>>a[i].y;
}

long double det(int k)
{
    long double ki=a[i].x*(a[k].y-a[k+1].y)+a[1].y*(a[k+1].x-a[k].x)+a[k].x*a[k+1].y-a[k+1].x*a[k].y;
    if(ki<0)
        ki*=-1;
    return ki/2;
}

int suma()
{
    long double s=0;
    for(int i=2;i<n;i++)
        s+=det(i);
    return s;
}

int main()
{
    citire();
    g<<setprecision(6)<<fixed<<suma();
    return 0;
}