s, e, a = map(int, input().split())
for i in range(a*11):
#print(i+1,s) #test
if s >= e:
print(i+1)
break
if (i+1)%9 == 0 or (i+1)%10 == 0: #take rest
continue
elif (i+1)%11 == 0: #unsalable
if s > e: (這裡如果這樣寫10 10 1的側資,會輸出0行為甚麼?)(我知道 s < e 才是對的)
a-=1
if a == 0:
print("unsalable")
break
s += s//10
continue
elif (i+1)%3 == 0: #use fertil
s += s//3
else: #normal water
s += s//10
# 10 10 1 error