#include <fstream>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double arie,puncte[2][100001];
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>puncte[0][i]>>puncte[1][i];
    puncte[0][0]=puncte[0][n];
    puncte[1][0]=puncte[1][n];
    for(i=0;i<n;i++)
        arie=arie+(puncte[0][i]*puncte[1][i+1]-puncte[0][i+1]*puncte[1][i]);
    g<<arie/2.0;
    return 0;
}