#include <bits/stdc++.h>
using namespace std;
struct my{
    double x,y;
}v[100010];double rs;
int main(){
	ifstream cin("aria.in");
	ofstream cout("aria.out");
    int n;cin>>n;
    for(int i(1);i<=n;++i)
        cin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(int i(1);i<=n;++i)
        rs+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    cout<<setprecision(6)<<fixed<<rs/2<<"\n";
    return 0;
}