#include <bits/stdc++.h>

using namespace std;

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

const int DIM = 1e5 + 7;

int x[DIM];
int y[DIM];

int main()
{
    int n;
    in >> n;

    for(int i = 1; i <= n; i++)
        in >> x[i] >> y[i];

    x[n + 1] = x[1];
    y[n + 1] = y[1];

    long long ans = 0;

    for(int i = 1; i <= n + 1; i++)
        ans += x[i] * y[i + 1] - x[i + 1] * y[i];

    ans = abs(ans);

    out << ans / 2;

    if(ans & 1)
        out << ".5";
}