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

from collections import defaultdict

a,b=LI()
n=II()
dp={1:0}
st=[(n,1)]
x2p=defaultdict(list)
while st:
    # print(st,dp,x2p)
    x,f=st.pop()
    if x in dp:continue
    if f:
        st.append((x,0))
        l = 2
        while 1:
            p = (x+l-1)//l
            if p < 1: break
            x2p[x].append(p)
            if p not in dp:st.append((p,1))
            if p == 1: break
            l = (x+p-2)//(p-1)
    else:
        cur=inf
        for p in x2p[x]:
            cur=min(cur,dp[p]+a+((x+p-1)//p-1)*b)
        dp[x]=cur

print(dp[n])