# Enter your code here. Read input from STDIN. Print output to STDOUT
lst=list()
i=raw_input()
lst=i.split()
if len(lst)==3:
	a=float(lst[0])
	b=float(lst[1])
	n=int(lst[2])
        grow=((0.5*a)+(0.5*b))*n
        rate=grow%(10**9+7)
        print long(rate)