c1,c2,n = input().split()
n = int(n)
if c1==c2:
	if n==1:
		print(c1)
	else:
		print(c1+"a"*(n-2)+c2)
else:
	if n==1:
		print(-1)
	else:
		print(c1+"a"*(n-2)+c2)