#include <cstdio>
#include <cmath>

using namespace std;

struct ar
{
    int x;
    int y;
} a[100000];

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

    int n;
    double sum=0;
    scanf("%d", &n);
    for(int i=0;i<n;i++)
    {
        scanf("%d %d", &a[i].x, &a[i].y);
        if(i>0)
            sum=sum+((a[i-1].x*a[i].y)-(a[i].x*a[i-1].y));
    }
    sum=(sum+((a[n-1].x*a[0].y)-(a[0].x*a[n-1].y)))/2;
    printf("%lf", fabs (sum / 1));
        return 0;
}