summation formula

I am trying to make a summation of a range of numbers in a data set based on a specific criteria.

In column 1 represents a person specified by a number (1,2,3...)
Column 2 represents the number of miles a person ran
So in column 3 I want to have a summation of the total mileage ran by each person on the row with the last entry of the person.

What I started to do was simply sum the range (cell C12 =sum(B1:B:12))
The problem with this is that I have many individuals in column 1 and they are not all the same length so I can't just use the same summation formula and copy it.
Is their a formula I could us to make it so all the cells return no value in column C except for the last one of the person other than manually typing in =sum(range) on the last cell of each individual?

I hope what I am trying to do is clear.


I have attached an example if my words were not clear.
Thanks for the help!!