#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
double aria=0;
int main()
{
    int n,i,x,y,a,b,z,t;
    f>>n;
    f>>x>>y;
    z=x;
    t=y;
    for(i=2;i<=n;i++)
    {
        f>>a>>b;
        aria += x*b-a*y;
        x=a;y=b;
    }
    aria+=z*b-a*t;
    aria = (double)aria;
    g<<fabs(aria);
}