#include <cstdio>
#include <cmath>

struct point
{
    double x;
    double y;
};

double CrossProduct(const point &p1, const point &p2)
{
    return p1.x * p2.y - p1.y * p2.x;
}
double PolygonArea(const point vertices[], int N)
{
    double sum = 0.0;

    for(int i = 1; i <= N; i++)
    {
        sum += CrossProduct(vertices[i], vertices[(i == N) ? 1 : i + 1]);
    }

    return fabs(sum) / 2.0;
}

int N;
point vertices[100001];

int main()
{

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

    scanf("%d", &N);

    for(int i = N; i >= 1; i--)
    {
        scanf("%lf %lf", &vertices[i].x, &vertices[i].y);
    }

    printf("%.5lf", PolygonArea(vertices, N));

    return 0;
}