a,b,x = map(int,input().split())
dan = x%3
ju = x//3

if a<=b/3:
    min = x*a
elif a>b and dan != 0:
    min = (ju+1)*b
elif a>b and dan == 0:
    min = (ju)*b
else:
    min = dan*a + ju*b
print(min)