#include <iostream>
#include <fstream>
using namespace std;
long double a[100005], b[100005], s;
int i, n;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n;
    for(i=1; i<=n; i++) f >> a[i] >> b[i];
    a[n+1]=a[i]; b[n+1]=b[1];
    for(i=1; i<=n; i++) s+=(a[i]*b[i+1]-a[i+1]*b[i]);
    s/=2.0;
    g <<s;
    return 0;
}