#include <fstream>
#include <cmath>

using namespace std;

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

struct Punct
{
    float x, y;
};

float AriePoligon(int N, Punct P[])
{
    P[N + 1] = P[1];
    int i;
    float s = 0;
    for( i = 1; i <= N; ++i )
        s += P[i].x * P[i + 1].y - P[i + 1].x * P[i].y;

    return fabs(s * 0.5);
}

int N;
Punct P[100001];

int main()
{
    fin >> N;
    for( int i = 1; i <= N; ++i )
        fin >> P[i].x >> P[i].y;

    fout << AriePoligon(N, P);
    return 0;
}