how many 4 - digit positive integers are there , where each digit is positive , and no 3 adjacent digits are same ?
first digit . . 9 posibilities second digit , 8 possibilities third digit , 7 possibilities fourth digit , 7 possibilities . 9 * 8 * 7 * 7 = 3528 . b
multiply(multiply(add(const_4, const_3), add(const_4, const_3)), multiply(add(4, 4), multiply(3, 3)))