We are given an array A of length N <= 500000.

We need to find the smallest valid number. A number V is valid if the following conditions are met:

- in the prefix of length V of the array, atleast one number >= V must appear.
- If we colour a window of length V starting from each position of appearance of a number >= V, all the elements except possibly some prefix are coloured. In other words, for all 'i' such that A[i] >= V, we colour all elements A[i], A[i+1], ..., A[min(i+V-1, N-1)]. After all such colouring operations, the entire array except possibly some prefix of the array should be coloured.

Any help would be appreciated, thanks a lot!

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