#include <bits/stdc++.h>
#define int long double
using namespace std;



signed main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    int n, a, b, x, y;
    cin >> n >> a >> b;
    int aa, bb;
    aa = a;
    bb = b;
    int ans = 0;
    for(int i = 2; i <= n; i++){
        cin >> x >> y;
        ans += a * y - b * x;
        a = x;
        b = y;
    }
    ans += x * bb - y * aa;
    ans /= 2;
    ans = abs(ans);
    cout << fixed << setprecision(5) << ans;
    return 0;
}