I have a small long format dataset below.
* Example generated by -dataex-. For more info, type help dataex
clear
input byte (gender period pr)
1 1 11
1 1 11
1 1 11
1 2 15
1 2 15
1 2 15
2 1 11
2 1 11
2 1 11
2 2 12
2 2 12
2 2 12
end

I want to create a new variable sum_pr,
which equals to the sum of the first value within gender and period like below:
* Example generated by -dataex-. For more info, type help dataex
clear
input byte (gender period pr sum_pr)
1 1 11 26
1 1 11 26
1 1 11 26
1 2 15 26
1 2 15 26
1 2 15 26
2 1 11 23
2 1 11 23
2 1 11 23
2 2 12 23
2 2 12 23
2 2 12 23
end

How can I use Stata code to achieve it?
Thank you!