a,b,x = map(int,input().split())
if 3*a>b:
    quotient,remainder = divmod(x,3)
    res = b*quotient+a*remainder # 尽量买3的倍数，剩下的单买
    res2 = b*(quotient+1)# 直接多买一组
    print(min(res,res2))
else:
    print(x*a)