#include <iostream>
#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
 double x,y;
}a[100005];
int abs(int x)
{
    if(x>0)
        return x;
    else
        return -x;
}
double s;
int i,n;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>a[i].x;
        f>>a[i].y;
    }
    a[n+1]=a[1];
    s=0;
    for(i=1;i<=n;i++)
        s+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);

    s/=2.0;
    if(s<0)
        s=-s;
    g<<s;


    return 0;
}