while True:
    try:
        n = int(input())
        if n == 1:
            print(0)
        elif n == 2:
            print(1)
        else:
            sum = 1
            for i in range(3, n+1):
                sum += 2**(i-2)+2**(i-1)
            print(sum % (10**9+7))
    except:
        break

暴力解法，找规律