A, B = map(int, input().split())
N = int(input())

ret: int = (A + B) * 30

if N == 1:
    ret = 0

for num_a in range(1, 31):
    ng, ok = 0, N - 1

    while ok - ng > 1:
        c: int = (ok + ng) // 2
        x, md = divmod(c, num_a)
        current = 1
        for t in range(num_a):
            y = x + (t < md)
            current *= 1 + y
            if current >= N:
                break
        if current >= N:
            ok = c
        else:
            ng = c

    ret = min(ret, num_a * A + ok * B)

print(ret)