#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long long n,i;
double s;
struct coord
{
   int x,y;
}a[1000001];
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>a[i].x>>a[i].y;
    a[1]=a[n+1];
    for(i=1;i<=n;i++)
        s=s+(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    if(s<0)
        g<<-1;
    else
        g<<s;
    return 0;
}