#include <cstdio>
#include <utility>
#include <cmath>

using namespace std;

int n;
double sol;
pair < int, int > pos[100005];

int main()
{
	FILE *in, *out;
	in = freopen("aria.in", "r", stdin);
	out = freopen("aria.out", "w", stdout);

	scanf("%d", &n);
	for (int i = 1; i <= n; i++) scanf("%d%d", &pos[i].first, &pos[i].second);
	fclose(in);

	for (int i = 1; i < n; i++) sol += (pos[i].first * pos[i + 1].second - pos[i + 1].first * pos[i].second);
	sol += (pos[n].first * pos[1].second - pos[1].first * pos[n].second);
	sol = abs(sol / 2);

	printf("%.5f", sol);
	fclose(out);

    return 0;
}