#include <bits/stdc++.h>

using namespace std;

struct punct{
    double x;
    double y;
};
punct t[100005];
int n,i;
double s;

int main() 
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    cin>>n;
    for (i=1; i<=n; i++)
        cin>>t[i].x>>t[i].y;
        
    t[n+1]=t[1];
    
    for (i=1; i<=n; i++)
    {
        s+=t[i].x*t[i+1].y-t[i+1].x*t[i].y;
    }
    cout<<setprecision(5)<<fixed<<s/2;
}