Problem:
Score : 700 points

Problem Statement

We have a tree with N vertices. Vertex 1 is the root of the tree, and the parent of Vertex i (2 \leq i \leq N) is Vertex P_i.

To each vertex in the tree, Snuke will allocate a color, either black or white, and a non-negative integer weight.

Snuke has a favorite integer sequence, X_1, X_2, ..., X_N, so he wants to allocate colors and weights so that the following condition is satisfied for all v.

The total weight of the vertices with the same color as v among the vertices contained in the subtree whose root is v, is X_v.

Here, the subtree whose root is v is the tree consisting of Vertex v and all of its descendants.

Determine whether it is possible to allocate colors and weights in this way.

Constraints

1 \leq N \leq 1 000

1 \leq P_i \leq i - 1

0 \leq X_i \leq 5 000

Inputs

Input is given from Standard Input in the following format:

N
P_2 P_3 ... P_N
X_1 X_2 ... X_N

Outputs

If it is possible to allocate colors and weights to the vertices so that the condition is satisfied, print POSSIBLE; otherwise, print IMPOSSIBLE.

Sample Input 1

3
1 1
4 3 2

Sample Output 1

POSSIBLE

For example, the following allocation satisfies the condition:

Set the color of Vertex 1 to white and its weight to 2.

Set the color of Vertex 2 to black and its weight to 3.

Set the color of Vertex 3 to white and its weight to 2.

There are also other possible allocations.

Sample Input 2

3
1 2
1 2 3

Sample Output 2

IMPOSSIBLE

If the same color is allocated to Vertex 2 and Vertex 3, Vertex 2 cannot be allocated a non-negative weight.

If different colors are allocated to Vertex 2 and 3, no matter which color is allocated to Vertex 1, it cannot be allocated a non-negative weight.

Thus, there exists no allocation of colors and weights that satisfies the condition.

Sample Input 3

8
1 1 1 3 4 5 5
4 1 6 2 2 1 3 3

Sample Output 3

POSSIBLE

Sample Input 4

1

0

Sample Output 4

POSSIBLE

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e5+1;


int n,fa[N],tot[N],fr[N],a[N],b[N],c[N],cnt,head[N];

struct nd{int ne,to;}e[N];

void in(int x,int y){e[++cnt].to=y;e[cnt].ne=head[x];head[x]=cnt;}
int w1[5001],q[N];
void dfs(int x)
{
	for(int i=head[x];i;i=e[i].ne)dfs(e[i].to);
	if(!fr[x])return;
	q[0]=0;
	int ret=tot[x],tot1=0;
	for(int i=head[x];i;i=e[i].ne)
	{
		ret-=min(a[e[i].to],b[e[i].to]);
		q[++q[0]]=abs(a[e[i].to]-b[e[i].to]);
		tot1+=a[e[i].to]+b[e[i].to];
	}
	if(ret<0){cout<<"IMPOSSIBLE";exit(0);}
	memset(w1,0,sizeof(w1));
	w1[0]=1;
	for(int i=1;i<=q[0];++i)
	for(int t=5000;t>=q[i];--t)
	if(w1[t-q[i]]==1)w1[t]=1;
	for(int i=ret;i>=0;--i)
	if(w1[i]==1){ret-=i;break;}
	a[x]=tot[x];b[x]=tot1+ret-a[x];
	return;
}

int main()
{
	scanf("%d",&n);
	for(int i=2;i<=n;++i)
	scanf("%d",&fa[i]),in(fa[i],i),fr[fa[i]]++;
	for(int i=1;i<=n;++i)
	{
		scanf("%d",&tot[i]);
		if(!fr[i])a[i]=tot[i],b[i]=0;
	}
	dfs(1);
	cout<<"POSSIBLE";


}