#include <cstdio>

#define NMAX 100005

struct punct
{
    double x, y;
};

double arie_trapez(punct A, punct B)
{
    double lm = (A.y + B.y) / 2, h = A.x - B.x;
    return lm * h;
}

int N;
double arie;
punct A[NMAX];

void citire()
{
    scanf("%d", &N);

    for (int i = 1; i <= N; ++i)
        scanf("%lf %lf", &A[i].x, &A[i].y);
    A[N + 1] = A[1];
}

void rezolvare()
{
    double x;

    for (int i = 1; i <= N; ++i)
    {
        x = arie_trapez(A[i], A[i + 1]);
        arie += x;
    }

    if (arie < 0)
        arie = -arie;

    printf("%lf", arie);
}

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    citire();
    rezolvare();

    return 0;
}