#include <iostream>
#include <fstream>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct{long double x; long double y;} v[100002];
long double a,a2,x1,x2,y1,y2;
long long n,i,j;

int main()
{
    f>>n;
    for(i=1;i<=n;i++) f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    a=0;
    for(i=1;i<=n;i++)
    {
        x1=v[i].x;
        x2=v[i+1].x;
        y1=v[i].y;
        y2=v[i+1].y;
        a2=(x1*y2-x2*y1)/2;
        a+=a2;
    }
    if(a<0) a=-a;
    g<<a<<'\n';
    return 0;
}