# Write a function to find the nth jacobsthal number.
def jacobsthal_num(n):
    dp = [0] * (n + 1)
    dp[0] = 0
    dp[1] = 1
    for i in range(2, n + 1):
        dp[i] = dp[i - 1] + 2 * dp[i - 2]
    return dp[n]
