 divide "a" and "b", since they leave a remainder.Since "r""N"−1 is a common divisor of "a" and "b", "r""N"−1 ≤ "g".In the second step, any natural number "c" that divides both "a" and "b" (in other words, any common divisor of "a" and "b") divides the remainders "r""k".