1 million = 1,000,000

The successor of a number is one more than that number and the predecessor of a number is one less than that number.

The successor of 1,000,000 = 1000000 + 1 = 1000001
The predecessor of 1,000,000 = 1000000 - 1 = 999999

The difference between the successor and the predecessor of 1,000,000 = 1000001 - 999999 = 2.
In fact the difference between the successor and predecessor of any number will always be 2 !! Try it yourself