I have a data frame with five thousands rows. I need to create a new column with a unique identifier based on column "gender", then the number 21, and a sequential number starting on 0001. It is important that the sequential number restarts with a different letter in column "gender" (gender + 21 + seq#).

```
df <- data_frame(
  name = c("A", "B", "C", "D", "E", "F", "G", "H", "I"),
  gender = c("F", "F", "F", "M","M","F","M","F","F")
)

df
name  gender
  <chr> <chr> 
1 A     F     
2 B     F     
3 C     F     
4 D     M     
5 E     M     
6 F     F     
7 G     M     
8 H     F     
9 I     F

```

With unique identifier:

```
df
name  gender  id
1 A     F     F210001
2 B     F     F210002
3 C     F     F210003
4 D     M     M210001
5 E     M     M210002
6 F     F     F210004
7 G     M     M210003
8 H     F     F210005
9 I     F     F210006

```

Any help on how to achieve this will be very appreciated.