#include <cstdio>
using namespace std;
int i,n;
float s;
struct rec{float x,y;}a[100010];
float abs(float k)
{
    if(k<0)return -k;
    else return k;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%f %f",&a[i].x,&a[i].y);
    }
    a[n+1]=a[1];
    for(i=1;i<=n;i++)
    {
        s+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;
    }
    s=s/2;
    printf("%f",abs(s));
    return 0;
}