Constant in formula

Hello,

I am trying to use a input cel in excell

1) On my excel sheet I have data on A1,A2,A3,A4,A5 containing the amount of ml ink being used.

2) on cell B1 I have the input of Priceper ml that I want the user to put in.

3) on C1 I have the formula of "=ROUNDDOWN(SUM(A1*B1),2)"

now if I drag C1 down to C5 I want in my formula A1 to count from A1 to A5 but I dont want B1 to count but be constant.

My problem is when I drag down from C1 to C5 that in my formula B1 also counts from B1 to B5 .

How can I have B1 as input and remain constant in the formula