#include <cstdio>
#include <iostream>
#include <fstream>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <cstdlib>
#include <ctype.h>
#include <cstring>
#include <string>
#include <ctime>
#include <cassert>
#include <utility>

#define LIM 100005

using namespace std;

struct Point{
    double x, y;
    Point() {}
    Point(double X, double Y) {
        x = X;
        y = Y;
    }
};

Point v[LIM];

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

    int n;
    cin >> n;
    for(int i = 0; i < n; i++) {
        cin >> v[i].x >> v[i].y;
    }

    v[n] = v[0];
    double s = 0;
    for(int i = 0; i < n; i++) {
        s += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);
    }

    cout.precision(6);
    cout << fixed << (1 / 2.0) * s;

	return 0;
}