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

struct punct
{
     double x, y;

} v[100005];

int n;
double sol;

int main ()
{
    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);

    cin>>n;
    for (int i=0;i<n;++i) cin>>v[i].x>>v[i].y;
    v[n] = v[0];

    for (int i=0;i<n;++i)  sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    cout<<fabs(sol/2.0);
}