#include <fstream>

using namespace std;


FILE * fout;
FILE * fin;

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

int main()
{
    fin = fopen("aria.in", "r");
    fout = fopen ("aria.out","w");
    fscanf(fin, "%lf", &n);
    fscanf(fin, "%lf %lf", &x, &y);
    ss=x;
    st=y;
    aria=0;
    for (i=2; i<=n; i++)
    {
        fscanf(fin, "%lf %lf", &a, &b);
        aria+=x*b-y*a;
        x=a;
        y=b;
    }
    aria+=a*st-b*ss;
    fprintf(fout, "%lf", aria/2.0);
    return 0;
}