#include <bits/stdc++.h>

using namespace std;

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


long double x[100005],y[100005];

int n;
long double aria=0;

int main()
{
    f>>n;
    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++)
    {
        aria = aria + x[i] * y[i+1] - x[i+1] * y[i];
    }

    g<< setprecision(5) << fixed << aria/2;
}