import sys

# sys.setrecursionlimit(1000006)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

def iroot(a, e):
    r = round(a**(1/e))
    while r**e > a: r -= 1
    return r

a, b = LI()
n = II()

if n == 1:
    print(0)
    exit()

ans = inf
for x in range(1, 31):
    y = iroot(n, x)
    s = y**x
    cnt = 0
    while s < n:
        s = s//y*(y+1)
        cnt += 1
    ans = min(ans, x*a+((y-1)*x+cnt)*b)
    if y < 2: break
print(ans)