#include <bits/stdc++.h>

using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");

struct Punct
{
    long double x, y;
}v[100005];

bool crt ( Punct a, Punct b );

 long double ariaTriunghiului ( Punct a, Punct b, Punct c )
{
    b.x =b.x- a.x;
    b.y =b.y- a.y;
    c.x =c.x- a.x;
    c.y =c.y- a.y;
    a.x = a.y = 0;

    return ( b.x * c.y - b.y * c.x ) / 2;
}

int main()
{
    int n, i;
    long double aria = 0;

    fin >> n;
    for ( i = 1 ; i <= n ; i++ ) fin >> v[i].x >> v[i].y;

    for ( i = 2 ; i < n ; i++ ) aria =aria+ ariaTriunghiului ( v[1], v[i], v[i+1] );

    fout << setprecision ( 5 ) << fixed << aria;

    return 0;
}