#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct puncte{
    double x, y;
}v[100200];

int n;
double s;

double det(puncte a,puncte b,puncte c){
    double X1=a.x, Y1=a.y;
    double X2=b.x, Y2=b.y;
    double X3=c.x, Y3=c.y;
    return (X2-X1)*(Y3-Y1)+(Y2-Y1)*(X3-X1);
}

int32_t main()
{
    f>>n;
    for(int i=1; i<=n; i++)
        f>>v[i].x>>v[i].y;
    for(int i=1; i<n; i++)
        s+=v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    g<<fixed<<setprecision(5)<<abs(s)/2.0<<'\n';
    return 0;
}