#include <bits/stdc++.h>
using namespace std;

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

const int MAXN = 1e5;
long double X[MAXN + 5], Y[MAXN + 5];
int N;
long double aria;
int main()
{
    fin >> N;
    for(int i = 0; i < N; ++i)
        fin >> X[i] >> Y[i];
    X[N] = X[0],
    Y[N] = Y[0];
    for(int i = 0; i < N; ++i) {
        aria += X[i] * Y[i + 1] - X[i + 1] * Y[i];
    }
    fout << fixed << setprecision(5) << aria / 2.0;
    return 0;
}