#include <bits/stdc++.h>
using namespace std;

struct punct
{
    long double x,y;
}a[100005];

int n;
long double aria;
ifstream f("aria.in");
ofstream g("aria.out");

int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>a[i].x>>a[i].y;
    }
    a[n+1]=a[1];
    for(int i=1;i<=n;i++)
        aria=aria+(a[i].x*a[i+1].y-a[i].y*a[i+1].x);
    aria=aria/2;
    if(aria<0) aria=-aria;
    g<<setprecision(6)<<fixed<<aria<<'\n';

    return 0;
}