#include<queue>
#include<math.h>
#include<stdio.h>
#include<string.h>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
#define N 1234567
#define M 12

double x[N];
double y[N];

int main()
{
    int n;
    while(cin>>n && n)
    {
        double sum = 0;
        for(int i = 1; i <= n; i++)
        {
            scanf("%lf %lf",&x[i], &y[i]);
        }
        for(int i = 1; i <= n-1; i++)
        {
            sum += (x[i]*y[i+1] - x[i+1]*y[i]);
        }
        sum += (x[n]*y[1] - x[1]*y[n]);

        printf("%.1f\n",0.5 * sum);

    }
    return 0;
}