View Code of Problem 4055

#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<cstdio>
#define N 1000010
using namespace std;
typedef long long ll;
struct dian{
	ll x,y;
}p[200];
double fun(int n){
	double s;
	int i,j;
	s=0;
	for(i=0,j=1;i<n;i++,j++){
		j=j%n;
		s+=p[i].x*p[j].y-p[i].y*p[j].x;
	}
	s=fabs(s*0.5);
	return s;
}
 
int main(){
	int n;
	while(cin>>n,n!=0){
		for(int i=0;i<n;i++){
			cin>>p[i].x>>p[i].y;
		}
	    printf("%.1f\n",fun(n));
	}
 
		
	return 0;
}