#include <cstdio>
#include <iostream>
#include <vector>
#include <queue>
#include <utility>
#include <algorithm>
#include <functional>
#include <string>
#include <cstring>
#include <cmath>
#include <map>
#include <set>
#include <bitset>
#include <stack>
#include <iomanip>
#define MOD 1000000000
#define NMAX 1000005
#define INF (1<<30)

using namespace std;

FILE *fin = freopen("aria.in", "r", stdin);
FILE *fout = freopen("aria.out", "w", stdout);

typedef pair<short, short> pii;

double x[NMAX], y[NMAX];

int main() {
	int n, i;
	double aria = 0;

	cin >> n;

	for (i = 0; i < n; ++i)
		cin >> x[i] >> y[i];

	x[n] = x[0];
	y[n] = y[0];
	for (i = 0; i < n; ++i)
		aria += (x[i] * y[i + 1] - y[i] * x[i + 1]);

	cout << setprecision(8) << 0.5*aria;

	return 0;
}