#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int NMAX=100005;
int n,aria;
struct punct
{
    int x,y;
}v[NMAX];
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;i++)
        aria+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    g<<aria/2.0;
    return 0;
}