# include <cstdio>
# include <cmath>
# include <algorithm>
using namespace std;
#define x first
#define y second
typedef pair <double, double> punct;
const char *FIN = "aria.in", *FOU = "aria.out";
const int MAX = 100005;

punct vec[MAX];
int N;

inline long double arie (punct *vec, int sz) {
    long double sol = 0;
    vec[sz++] = vec[0]; // vec[N] = vec[0]
    for (int i = 0; i + 1 < sz; ++i)
        sol += (long double) (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
    return fabs (sol * 0.5);
}

int main (void) {
    freopen (FIN, "r", stdin);
    freopen (FOU, "w", stdout);

    scanf ("%d", &N);
    for (int i = 0; i < N; ++i)
        scanf ("%lf %lf", &vec[i].x, &vec[i].y);
    printf ("%Lf", arie (vec, N));
}