#include <cstdio>
#include <cmath>

using namespace std;

const int Max = 100007;

struct yer
{
    double x, y;
}v[Max];

double sum;
int n;

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

    scanf("%d", &n);
    for(int i = 1; i <= n; ++i)
    {
        scanf("%lf%lf", &v[i].x, &v[i].y);
    }
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    for(int i = 1; i <= n; ++i)
    {
        sum+= v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    }
    printf("%lf", abs(sum) / 2.0);

    return 0;
}