#include <cstdio>

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

    int N, x[100002], y[100002];

    scanf("%d", &N);

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

    x[N + 1] = x[1];

    y[N + 1] = y[1];

    double area = 0;

    for(int i = 1; i <= N; i++)
    {
        area += x[i] * y[i + 1] - y[i] * x[i + 1];
    }

    printf("%.5lf", area / 2);

    return 0;
}