#include <iostream>
#include <fstream>

using namespace std;

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


int n,px,py;
long long s=0;


struct coord{

    int x,y;
}a[100005];

int main()
{
    f>>n;
    for(int i=0;i<n;i++)
    {
        f>>a[i].x>>a[i].y;
    }
    for(int i=0;i<n;i++)
    {
        s+=(a[i].x*a[i+1].y)-(a[i+1].x*a[i].y);

    }
    s+=(a[n-1].x*a[0].y)-(a[0].x*a[n-1].y);
    g<<(float)s/2;
    return 0;
}