#include <bits/stdc++.h>

#define Nmax 100000

using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");

struct Point
{
    int x, y;
};

int N;

Point A[1 + Nmax + 5];

long double answer = 0.0;

long double cross_product (Point a, Point b)
{
    return 1.0 * a.x * b.y - a.y * b.y;
}

int main()
{
    fin >> N;
    for (int i = 1; i <= N; ++i)
        fin >> A[i].x >> A[i].y;
    for (int i = 1; i <= N; ++i)
        answer += (cross_product (A[i], A[(1 + i) % N]));
    fout << answer;
    return 0;
}