#include <fstream>
#define NMAX 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

double x[NMAX],y[NMAX];
int main()
{
    int N;
    fin>>N;
    for(int i = 1; i<=N; i++)
        fin>>x[i]>>y[i];

    x[0] = x[N], y[0] = y[N];
    double aria = 0;
    for(int i = 0; i<=N;i++)
    {
        aria += 1.0*(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    aria/=2.0;

    fout<<aria<<'\n';
    return 0;

}