#include<bits/stdc++.h>
#define pdd pair<double,double>
#define x first
#define y second
#define N 100030
using namespace std;

double det(pdd A, pdd B, pdd C) {
    return (A.x*B.y+B.x*C.y+C.x*A.y-B.y*C.x-C.y*A.x-A.y*B.x);
}
pdd a[N];
double rs;
int n;

int main() {
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    cin>>n;
    for (int i=1; i<=n; ++i) cin>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    for (int i=1; i<=n; ++i) rs+=det({0,0},a[i],a[i+1]);
    cout<<fixed<<setprecision(10)<<0.5*rs;


    return 0;
}