#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N = 100010;
int n;
long double x[N], y[N], aria;
int main()
{
    f >> n ;
    for(int i = 1;i <= n;i++)
        f >> x[i] >> y[i];
    x[0] = x[n];
    y[0] = y[n];
    for(int i=1;i<=n;i++)
        aria += (x[i - 1] - x[i]) * (y[i - 1] + y[i]) / 2.0;
    if(aria < 0.0)
        aria = -aria;
    g << fixed << setprecision(6) << aria;
    return 0;
}