a, b, x = map(int, input().split())
def f(a, b, x):
    if x == 0:
        return 0
    if x == 1:
        return min(a, b)
    if x == 2:
        return min(a * 2, b)
    if x == 3:
        return min(a * 3, b)
    return f(a, b, 3) * (x // 3) + f(a, b, x % 3)

print(f(a, b, x))