#include <bits/stdc++.h>

using namespace std;

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

int N;
long long x[100005];
long long y[100005];

int main()
{
    double suma = 0;
    fin >> N;
    for (int i = 1; i <= N; ++i)
        fin >> x[i] >> y[i];
    x[N + 1] = x[1];
    y[N + 1] = y[1];
    for (int i = 1; i <= N; ++i)
        suma = suma + x[i] * y[i + 1] - x[i + 1] * y[i];
    suma = suma / 2;
    fout << abs(suma);
    return 0;
}