#include <bits/stdc++.h>

using namespace std;

#define Point pair<long double,long double>
#define x first
#define y second

const int Nmax = 100000;

Point P[Nmax + 1];
int N;
long double xG, yG;

int main()
{
    ifstream in("aria.in");
    ofstream out("aria.out");

    in >> N;

    for ( int i = 0; i < N; ++i )
        in >> P[i].x >> P[i].y;

    P[N] = P[0];

    long double A = 0;

    for ( int i = 0; i < N; ++i )
        A += P[i].x * P[i + 1].y - P[i + 1].x * P[i].y;

    out << fixed << setprecision(10);
    out << abs(A / 2.0) << "\n";

    return 0;
}