#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
typedef long long lol;
lol n;
double ans;
struct Vector
{
    double x,y;
    Vector(){}
    Vector(double xx,double yy){x=xx;y=yy;}
    Vector operator+(Vector b)
    {
        Vector s;
        s.x=x+b.x;s.y=y+b.y;
        return s;
    }
    Vector operator-(Vector b)
    {
        Vector s;
        s.x=x-b.x;s.y=y-b.y;
        return s;
    }
    double operator*(Vector b)
    {
        return x*b.y-y*b.x;
    }
    double operator^(Vector b)
    {
        return x*b.x+y*b.y;
    }
}a,b,c;
lol gi()
{
    lol ans=0,f=1;
    char i=getchar();
    while(i<‘0‘||i>‘9‘){if(i==‘-‘)f=-1;i=getchar();}
    while(i>=‘0‘&&i<=‘9‘){ans=ans*10+i-‘0‘;i=getchar();}
    return ans*f;
}
int main()
{
    while(1)
    {
        n=gi();
        if(n==0)break;
        ans=0;
        scanf("%lf%lf%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y,&c.x,&c.y);
        ans+=(b-a)*(c-a);
        for(lol i=4;i<=n;i++)
        {
            b=c;
            scanf("%lf%lf",&c.x,&c.y);
            ans+=(b-a)*(c-a);
        }
        ans/=2;
        printf("%.1lf\n",ans);
    }
    return 0;
}