Even odd

 l,h = map(int,input().split())

k = int(input())

lst=[]

for i in range(l,h+1):

    lst.append(i)

e =[]

o=[]

for i in lst:

    if i%2==0:

        e.append(i)

    else:

        o.append(i)

el = len(e)

ol = len(o)

res = 0.5*((ol+el)*k + (el-ol)*k)

print(int(res))