#include <bits/stdc++.h>
#define MAXN 100000
using namespace std;
struct coord {
    long double x , y;
};
coord v[MAXN + 1];
int main() {
    ifstream cin( "aria.in" );
    ofstream cout( "aria.out" );
    int n , i;
    long double ans;
    ans = 0;
    cin >> n;
    for( i = 0 ; i < n ; i++ )
        cin >> v[i].x >> v[i].y;
    for( i = 0 ; i < n ; i++ )
        ans += v[i].x * v[( i + 1 ) % n].y - v[i].y * v[( i + 1 ) % n].x;
    ans = abs( ans );
    ans /= 2;
    cout << fixed << setprecision( 7 ) << ans << '\n';
    return 0;
}