#include <bits/stdc++.h>
#define N 100005

using namespace std;

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

pair<int,int> p[N] ;

int main()
{
    double s;
    int n , i ;
    fin >> n ;
    for ( i = 1 ; i <= n ; i++ )
        fin >> p[i].first >> p[i].second;
    p[n+1] = p[1] ;
    for ( i = 1 ; i <= n ; i++ )
        s = s + p[i].first*p[i+1].second - p[i+1].first*p[i].second ;
    s = s/2 ;
    fout << setprecision(5) << s ;
}