Forbidden character

Your task is to count how many substrings of a string do not contain the character a.

The time complexity of the algorithm should be O(n).

In a file forbidden.py, implement a function count that returns the desired count.

def count(s):
    # TODO

if __name__ == "__main__":
    print(count("aaa")) # 0
    print(count("saippuakauppias")) # 23
    print(count("x")) # 1
    print(count("aybabtu")) # 9