#include <bits/stdc++.h>

using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int n;
long double s=0,a,b,x,y,a1,b1;
int main()
{
    in>>n;
    in>>x>>y;
    a1=x;
    b1=y;
    for(int i=1;i<n;i++)
    {
        in>>a>>b;
        s+=0.5*(x*b-y*a);
        x=a;
        y=b;
    }
    s+=0.5*(x*b1-y*a1);
    if(s<0)
        s=s*-1;
    out<<fixed<<setprecision(6)<<s;
    return 0;
}