60.±1 Walk Ending at 4

★★★

combinatoricsCh. 1 — Classical Probability and Combinatorics444+ Problems in Probability

Let

x_0 = 0

and let

x_1, \dots, x_{10}

satisfy

|x_i - x_{i-1}| = 1

for

1 \le i \le 10

, with

x_{10} = 4

. How many such sequences are there?

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