import sys

while True:
	try:	
		line = sys.stdin.readline()
		if not line:
			break
		l = int(line.rstrip())
		a =  [int(k) for k in sys.stdin.readline().rstrip().split()]
		i = 0
		if l <= 1:
			print(1)
			continue

		d = a[1] - a[0]
		for j in range(1, l - 1):
			nd = a[j + 1] - a[j]
			if nd != d:
				i = j
				d = nd
						
		print(i + 1)

	except EOFError:
		break

exit()