#include <cstdio>

#define NMAX 100002

using namespace std;

struct punct
{
    double x;
    double y;
} a[NMAX];

int n;

void read()
{
    freopen("aria.in", "r", stdin);

    scanf("%d\n", &n);

    for(int i = 0; i < n; ++ i)
        scanf("%lf %lf\n", &a[i].x, &a[i].y);
}

double det(punct A, punct B, punct C)
{
    return A.x * B.y + A.y * C.x + B.x * C.y - B.y * C.x - A.y * B.x - A.x * C.y;
}

double solve()
{
    double A = 0;

    punct P = {0, 0};
    a[n] = a[0];

    for(int i = 1; i <= n; ++ i)
        A += det(a[i - 1], a[i], P) / 2;

    return A;
}

int main()
{
    read();

    freopen("aria.out", "w", stdout);
    printf("%lf\n", solve());

    return 0;
}