#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdio>
#include<cmath>
#include<string>
#include<queue>
#include<algorithm>
#include<stack>
#include<cstring>
#include<vector>
#include<list>
#include<bitset>
#include<set>
#include<map>
#include<time.h>
using namespace std;
#define LL long long
#define pi (4*atan(1.0))
#define eps 1e-8
#define bug(x)  cout<<"bug"<<x<<endl;
const int N=1e5+10,M=1e6+10,inf=1e9+10;
const LL INF=1e18+10,mod=1e9+7;

struct Point
{
    double x, y ;
} p[N];
int n ;
double Area( Point p0, Point p1, Point p2 )// 求三角形面积公式
{
    double area = 0 ;
    area =  p0.x * p1.y + p1.x * p2.y + p2.x * p0.y - p1.x * p0.y - p2.x * p1.y - p0.x * p2.y;
    return area / 2 ;  //另外在求解的过程中，不需要考虑点的输入顺序是顺时针还是逆时针，相除后就抵消了。
}
double xjhz()
{
    double sum_area = 0 ;
    for ( int i = 2 ; i < n ;  i++ )
    {
        double area = Area(p[0],p[i-1],p[i]) ;
        sum_area += area ;
    }
    return sum_area;
}
int main ()
{
    while(~scanf ( "%d", &n ) )
    {
        if(!n)break;
        for(int i=0; i<n; i++)
            scanf ( "%lf%lf", &p[i].x, &p[i].y );
        printf("%.1f\n",xjhz()) ;
    }
    return 0 ;
}