#include <bits/stdc++.h>

using namespace std;

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

int n,i;
long double A,x,y,new_x,new_y,first_x,first_y;

int main()
{
    f>>n;
    f>>x>>y;

    first_x=x;first_y=y;
    for(i=2;i<=n;i++)
    {
        f>>new_x>>new_y;

        /// determinant:
        ///   0     0   1 (asta e originea)
        ///   x     y   1 (punctul anterior)
        /// new_x new_y 1 (punctul curent)

        A += (x*new_y - y*new_x);

        x=new_x;y=new_y;
    }

    /// se face si dintre ultimul punct si primul
    A += (x*first_y - y*first_x);

    /// in final, se impare la 2.0
    A /= 2.0;

    g<<fixed<<setprecision(5)<<A;
    return 0;
}