a,b,x = map(int, input().split())
n = x // 3 
m = x - n * 3
sum = 0
if b <= a * 3:
    if a * m < b:
        sum = n * b + m * a
    else:
        sum = (n + 1) * b
elif b > 3 * a:
    sum = a * x
print(sum)