hbmy周赛1--B

 3 0 0 1 0 0 1
4 1 0 0 1 -1 0 0 -1
0 

 0.5
2.0 



#include <iostream>
#include <stdio.h>
#include <cmath>
using namespace std;

double mianji(int x1,int y1,int x2, int y2, int x3, int y3)
{
	int p = (x1-x2)*(y1-y3) - (y1-y2)*(x1-x3);
	double num = (double)p / 2;
	return num;
}
int main()
{
	int n;
	int a[110], b[110];
	while (cin >>n && n)
	{
		for (int i=0; i<n; i++)
		{
			cin >> a[i] >> b[i];
		}
		double result = 0;
		for (int i=2; i<n; i++)
		{
			result = result + mianji(a[0],b[0],a[i-1],b[i-1],a[i],b[i]);
		}
		printf("%.1lf\n",result);
	}
	return 0;
}