#include <bits/stdc++.h>

using namespace std;

int N;
long double rs;



struct punct{
   double x, y;  
};

punct a[100005];

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[0]=a[N];
    a[N+1]=a[1];
    for(int i=1; i<=N; ++i)
        rs+=a[i].x*(a[i+1].y-a[i-1].y);
    cout<<setprecision(5)<<fixed<<rs/2.0;
    return 0;
}