#include<bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
void print(long double x)
{
    x*=100000;
    x=floor(x);
    x/=100000;
    fout<<x;
    /*int n=x;
    if(n==x)
        cout<<".0";*/
    return;
}
struct point{
long double x;
long double y;
};
long double area(point a,point b,point c)
{
    a.x-=c.x;
    a.y-=c.y;
    b.x-=c.x;
    b.y-=c.y;
    return ((a.x*b.y)-(a.y*b.x))/2;
}
long double full_area(int n,point* v)
{
    long double sup=0;
    for(int i=2;i<n;i++)
    {
        sup+=area(v[1],v[i],v[i+1]);
        //cout<<sup<<'\n';
    }
    return abs(sup);
}
int main()
{
    point v[100001];
    int n,i;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].y>>v[i].x;
    long double x=full_area(n,v);
    print(x);
    return 0;
}