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Image: ubuntu2004
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Kernel: Python 3 (system-wide)
def a(n):
    if n == 1:
        return 1
    elif n == 2:
        return 2
    elif n == 3:
        return 3
    elif n == 4:
        return 4
    else:
        return a(n-1)+a(n-2)+a(n-4)

a(20)
36894

This would mean that there are 36,894 ways you can walk up a set of stairs when using steps 1,2, and 4.