#include <fstream>
#define Nmax 100005

using namespace std;

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

int n;
pair <float,float> v[Nmax];
float A;

int main()
{
    in>>n;
    for(int i=1;i<=n;i++)
        in>>v[i].first>>v[i].second;
    v[n]=v[1];
    for(int i=1; i<=n; i++)
        A += (v[i].first * v[i+1].second) - (v[i+1].first * v[i].second);

    out<<A;
    return 0;
}