dplyr: how to summarise previously counted values by groups?

I counted marker frequency by groups:

name <- c("seq1", "seq1", "seq1", "seq2", "seq2", "seq2")
marker <- c("k", "k", "t", "t", "k", "t")
df <- data.frame(name,marker)

df %>% group_by(name, marker) %>% count(.)
# A tibble: 4 x 3
# Groups:   name, marker [4]
  name  marker     n
  <chr> <chr>  <int>
1 seq1  k          2
2 seq1  t          1
3 seq2  k          1
4 seq2  t          2

Now I want to infer a column with sum for each seq (3 for seq1 and 3 for seq2), how to do that?

  name  marker     n    sum
  <chr> <chr>  <int>  <int>
1 seq1  k          2     3
2 seq1  t          1     3
3 seq2  k          1     3
4 seq2  t          2     3