sumif in ifelse condition R

I have a DT with multiple columns and I need to give a condition in ifelse and do the calculations accordingly. I want it to do count/sum(count) grouped by segment. Here is the DT

Segment  Count  Flag
A        23     Y
B        45     N
A        56     N 
B        212    Y

I want the fourth column as count per total count of the segment based on the flag so the out put should look something like this. I am leaving blank for N flag because I have different column to compute but in the exact same manner, so I will be able to use the IFELSE condition.

Segment  Count  Flag   Value
    A        23     Y  23/66
    B        45     N  
    A        56     N  
    B        212    Y  212/224
    A        43     Y  43/66
    B        12     Y  12/224

Any help is appreciated. Thanks!