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

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

pair<long double,long double> p[N] ;

int main()
{
    long double s=0;
    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/2.0;
    fout << fixed << setprecision(6) << abs(s);
    return 0;
}