#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

const int NMax = 1e5 + 1;
int n;
double x[NMax], y[NMax];

int main()
{
    f>>n;
    double s= 0, det;
    for(int i=1; i<=n;i++)
        f>>x[i]>>y[i];

    x[n+1] = x[1];
    y[n+1] = y[1];


    for(int i=1 ; i<=n ; i++)
    {
        det+=x[i]*y[i+1]-x[i+1]*y[i];

    }
    g<<fixed<<setprecision(6)<<(long double) det / 2.0f;
    return 0;
}