In a repeating sequence of integers the sum of any group of concurrent integers of the same length as the pattern will always be the same.
IE:
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 ...
5 + 1 + 2 + 3 + 4 = 15
1 + 2 + 3 + 4 + 5 = 15
4 + 5 + 1 + 2 + 3 = 15
Commutative property of addition