# include <algorithm>
# include <cstdio>
# include <cmath>
using namespace std;

# define x first
# define y second

typedef pair <float, float> PR;
const char *FIN = "aria.in", *FOU = "aria.out";
const int MAX = 300005;

int N;
PR vec[MAX];

inline float arie (PR *vec, int sz) {
    float sol = 0;
    vec[sz++] = vec[0];
    for (int i = 0; i + 1 < sz; ++i)
        sol += 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 ("%f %f", &vec[i].x, &vec[i].y);
    printf ("%f\n", arie (vec, N));
}