A
B C
D E F
G H I J
K L M N O
Each letter value (A to J) is the absolute difference of the 2 values below it.
Example: if K = 9 and L = 5, then G = K - L = 9 - 5 = -4 = 4
(absolute value means disregard minus sign)
Replace each letter with the integers 1 to 15. No repeats!