#include <iostream>
#include <bits/stdc++.h>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
struct coord
{
    double x,y;
}v[1005];
 double arie(coord a, coord b, coord c)
{
    return abs(a.x*b.y+b.x*c.y+c.x*a.y-c.x*b.y-a.x*c.y-b.x*a.y)/2;
}
int n;
double suma()
{   double s=0;
    for(int i=3; i<=n; ++i)
        s+=arie(v[1],v[i-1],v[i]);
    return s;
}
int main()
{

    in>>n;
    for(int i=1; i<=n; ++i)
        in>>v[i].x>>v[i].y;
    out<<fixed<<setprecision(5)<<suma();
    return 0;
}