minsteps.py

def min_steps(numbers):
	initial = numbers[0]
	residual = initial%2
	minimum = initial
	maximum = initial
	for num in numbers:
		if num%2 != residual:
			return -1
		minimum = num if num < minimum else minimum
		maximum = num if num > maximum else maximum
	return (maximum - minimum)/2


test = [11,3,7,1]
print(min_steps(test))