1. You are given a number n, representing the number of stairs in a staircase.
2. You are on the 0th step and are required to climb to the top.
3. You are given n numbers, where ith element's value represents - till how far from the step you
    could jump to in a single move.  You can of-course fewer number of steps in the move.
4. You are required to print the number of minimum moves in which you can reach the top of
    staircase.
Note -> If there is no path through the staircase print null.
Input Format
A number n
.. n more elements
Output Format
A number representing the number of ways to climb the stairs from 0 to top.
Constraints
0 <= n <= 20
0 <= n1, n2, .. <= 20
Sample Input
10
3
3
0
2
1
2
4
2
0
0
Sample Output
4

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