#include <fstream>

using namespace std;

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

float n, x, y, ss, st, a, b, aria, i;

int main()
{
    fin>>n;
    fin>>x>>y;
    ss=x;
    st=y;
    aria=0;
    for (i=1; i<=n; i++)
    {
        fin>>a>>b;
        aria+=b*x-a*y;
        x=a;
        y=b;
    }
    aria+=a*st-b*ss;
    fout<<aria/2;
    return 0;
}