dynamic filename string

Í need to store data in seperate files dependeng on some integer
count, e.g.,

Do i = 1, N
...
filename = ?
...
Open(unit=1, file=filename,action="write",status="replace")
...
Close(1)
End do


How can I update the filename, such that it become a dynamic string
depent on the index i as follows

i=1 -> filename="data1.txt"
i=2 -> filename="data2.txt"
...

Allan
.