To find the greatest common divisor (GCD) of two numbers, I need to factorize both of them and see what factors they have in common. I could try to use prime factorization, but that might take too long. A quicker way is to use the Euclidean algorithm, which says that the GCD of two numbers $a$ and $b$ is the same as the GCD of $b$ and $a \mod b$, where $\mod$ means the remainder after division. So, I can start by dividing the larger number by the smaller one and see what the remainder is. If I divide $654321$ by $543210$, I get a quotient of $1$ and a remainder of $111111$. This means that the GCD of $654321$ and $543210$ is the same as the GCD of $543210$ and $111111$. I can repeat this process until I get a remainder of zero, and then the last nonzero remainder will be the GCD. Dividing $543210$ by $111111$, I get a quotient of $4$ and a remainder of $98766$. Dividing $111111$ by $98766$, I get a quotient of $1$ and a remainder of $12345$. Dividing $98766$ by $12345$, I get a quotient of $8$ and a remainder of $6$. Dividing $12345$ by $6$, I get a quotient of $2057$ and a remainder of $3$. Dividing $6$ by $3$, I get a quotient of $2$ and a remainder of $0$. So, the last nonzero remainder is $3$, and that is the GCD of $654321$ and $543210$. 
What is the greatest common divisor of $654321$ and $543210$?