#include<cstdio>
#include<algorithm>
using namespace std;
double aria,a[100005],b[100005];
int n,i,j,k;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for (i=1;i<=n;i++) scanf("%lf %lf",&a[i],&b[i]);
    a[n+1]=a[1];b[n+1]=b[1];
    for (i=1;i<=n;i++) aria=aria+((a[i]*b[i+1])-(a[i+1]*b[i]))/2;
    if (aria<0) aria=-aria;
    printf("%lf\n",aria);
    return 0;
}