Consider an integer array $A$ of size $n>0$ with elements $A[1],\ldots,A[n]$, where the first $p$ elements are positive and the other $(n-p)$ elements are negative. The following algorithm calculates $p$ correctly by calling COMPUTE-P(A,n). What loop invariant is maintained for the while loop of the algorithm?

COMPUTE-P(A, len) 1 if A[1] < 0 return 0 2 left = 1, right = len 3 while left < right 4 m = ceiling((left+right)/2) 5 if A[m] > 0 6 left = m 7 else 8 right = m - 1 9 return left