def stepPerms(n):
    memo = [-1] * (n+1)
    res = countSteps(n, memo) % 10000000007
    return res

def countSteps(n, memo):
    if n == 0:
        return 1 # Stay where you are
    if n == 1:
        return 1 # There is only 1 way to reach on top by takine 1 step
    if n == 2: 
        return 2 # There are 2 ways 1+1 or 2
    if n == 3:
        return 4 # There are 4 ways 1+1+1 or 2+1 or 1+2 or 3
    if memo[n] != -1:
        return memo[n]
    memo[n] = countSteps(n-1, memo) + countSteps(n-2, memo) + countSteps(n-3, memo)
    return memo[n]