import math
x = input().split()
x, y, n = float(x[0]), float(x[1]), int(x[2])
eps = 0.04999999999

if x <= y:
    print(0)
    quit()

if x != 10.0:
    s = (x-0.00000001+eps) * n
else:
    s = x*n

y = y-0.00000001+eps

k = (s - y*n) / (y-1)
k = int(k)
if (s + k-1) / (n + k - 1) - y <= -0.0001:
    print(k-1)
elif (s + k) / (n + k) - y <= -0.0001:
    print(k)
elif  (s + k+1) / (n + k+1) - y <= -0.0001:
    print(k+1)
# WA 6