#include <bits/stdc++.h>

#define N 100005
using namespace std;

int n, i;
double s, x[N], y[N];

double surf(int i, int j)
{
	return (x[j] - x[1]) * (y[i] - y[1]) - (x[i] - x[1]) * (y[j] - y[1]);
}

int main()
{
	freopen ("aria.in", "r", stdin);
	freopen ("aria.out", "w", stdout);
	scanf("%i", &n);
	for (i = 1; i <= n; i++)
		scanf("%lf%lf", &x[i], &y[i]);

	for (i = 2; i <= n - 1; i++)
		s += surf(i, i + 1) / 2;

	printf("%lf", fabs(s));
	return 0;
}