#include <bits/stdc++.h>

using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
struct punct
{
    double x;
    double y;
};
double arie_tri (punct a, punct b)
{
    return a.x*b.y-b.x*a.y;
}
long double sol;
int n;
punct v[100001];
int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
        in>>v[i].x>>v[i].y;
    for(int i=1; i<n; i++)
        sol+=arie_tri(v[i],v[i+1]);
    sol+=(arie_tri(v[n],v[1]));
    sol/=2.0;
    out<<fixed<<sol;
    return 0;
}