codefoceA--New Year Transportation

 New Year Transportation

Time Limit:2000MS     Memory Limit:262144KB     64bit IO Format:%I64d & %I64u

Submit  Status

Description

New Year is coming in Line World! In this world, there are n cells numbered by integers from 1 to n, as a 1 × n board. People live in cells. However, it was hard to move between distinct cells, because of the difficulty of escaping the cell. People wanted to meet people who live in other cells.

So, user tncks0121 has made a transportation system to move between these cells, to celebrate the New Year. First, he thought of n - 1positive integers a₁, a₂, ..., a_n - 1. For every integer i where 1 ≤ i ≤ n - 1 the condition 1 ≤ a_i ≤ n - i holds. Next, he made n - 1portals, numbered by integers from 1 to n - 1. The i-th (1 ≤ i ≤ n - 1) portal connects cell i and cell (i + a_i), and one can travel from cell i to cell (i + a_i) using the i-th portal. Unfortunately, one cannot use the portal backwards, which means one cannot move from cell(i + a_i) to cell i using the i-th portal. It is easy to see that because of condition 1 ≤ a_i ≤ n - i one can't leave the Line World using portals.

Currently, I am standing at cell 1, and I want to go to cell t. However, I don't know whether it is possible to go there. Please determine whether I can go to cell t by only using the construted transportation system.

Input

The first line contains two space-separated integers n (3 ≤ n ≤ 3 × 10⁴) and t (2 ≤ t ≤ n) — the number of cells, and the index of the cell which I want to go to.

The second line contains n - 1 space-separated integers a₁, a₂, ..., a_n - 1 (1 ≤ a_i ≤ n - i). It is guaranteed, that using the given transportation system, one cannot leave the Line World.

Output

If I can go to cell t using the transportation system, print "YES". Otherwise, print "NO".

Sample Input

Input

8 4
1 2 1 2 1 2 1

Output

YES

Input

8 5
1 2 1 2 1 1 1

Output

NO

Hint

In the first sample, the visited cells are: 1, 2, 4; so we can successfully visit the cell 4.

In the second sample, the possible cells to visit are: 1, 2, 4, 6, 7, 8; so we can't visit the cell 5, which we want to visit.

输入两个数n,m;

输入n-1个数a[i]，m表示想要跳到的位置

i+a[i]表示能跳到的位置，判断是否能跳到目标位置

题解：从开始连跳，在不超出m的范围内，看是否能到达m

code:

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#include<cstdio>
#include<cstring>
int a[50001];
int main()
{
	int n,m;
	while(~scanf("%d%d",&n,&m))
	{
		for(int i=1;i<=n-1;i++)
		scanf("%d",&a[i]);
	
		int j=1;
		while(j<m)
		{
			j+=a[j];}
			if(j==m)
			printf("YES\n");
			else
			printf("NO\n");	
	}
	return 0;
 }