#include<bits/stdc++.h>
using namespace std;

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

int n;

struct pct
{
    int x,y;
}v[100002];

int det(pct a, pct b, pct c)
{
    return a.x*b.y+a.y*b.x*c.y+c.x*a.y-b.y*c.x-a.x*c.y-b.x*a.y;
}

int main()
{
    int i,aria=0;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>v[i].x>>v[i].y;
    }
    ///luam p de coordonate (0,0)
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
    {
        aria+=det({0,0},v[i],v[i+1]);
    }
    g<<aria;
    return 0;
}