#include <cstdio>
#define NMAX 100007
using namespace std;
FILE *fin, *fout;
int n;
double x[NMAX], y[NMAX], s;
int main()
{
    fin = freopen("aria.in", "r", stdin);
    fout = freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(int i = 1; i<= n; ++i) scanf("%lf %lf", &x[i], &y[i]);
    for(int i = 1; i< n; i++)
    {
        s += (x[i]*y[i+1] - x[i+1]*y[i])/2;
    }
    s += ((long double)x[n]*y[1] - (long double)x[1]*y[n])/2;
    printf("%.5lf\n", s);
    fclose(fin);
    fclose(fout);
    return 0;
}