#include <fstream>
#include <iomanip>

using namespace std;

ifstream cin ("aria.in");
ofstream cout ("aria.out");

#define N 100000

long double l[N+2], c[N+2];

int main()
{
    int n,i;
    long double s;
    cin >> n;
    for (i=1; i<=n; i++)
        cin >> l[i] >> c[i];
    l[n+1]=l[1];
    c[n+1]=c[1];
    s=0;
    for (i=1; i<=n+1; i++)
        s+=l[i]*c[i+1]-l[i+1]*c[i];
    cout << fixed << setprecision(5) << abs(s)/2.0 << '\n';
    return 0;
}