#include <bits/stdc++.h>

using namespace std;

int const nmax = 1e5 + 5;

int n;
long double A[nmax] , B[nmax];
long double ar = 0;


int main()
{
    freopen("aria.in" , "r" , stdin);
    freopen("aria.out" , "w" ,stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> n;
    for(int i = 1;i <= n; ++i)cin >> A[i] >> B[i];
    A[n + 1] = A[1];
    B[n + 1] = B[1];
    for(int i = 1;i <= n; ++i)
        ar += A[i] * B[i + 1] - A[i + 1] * B[i];
    cout << fixed << setprecision(5) << ar / 2.0;
}