Calculate difference between multiple changing columns in R

I have a dataframe (df) in R with 7 columns

I want to add an 8th column called 'difference' that gives me the growth rate for each individual, which would be (size 3 - size 1) / (year 3 - year 1). I was able to do that with this code:

df2 <- df %>%
  mutate(difference = (df$Size3 -df$Size1)/(df$Year3 - df$Year1))

However, some individuals only have year 1 and year 2. How can I write a statement saying if Year3 = NA, then make the 'difference' column from (Size2 - Size1) / (Year2 - Year1)?

Desired output:

You can try ifelse

transform(
  df,
  Difference = ifelse(
    is.na(Year3),
    (Size2 - Size1) / (Year2 - Year1),
    (Size3 - Size1) / (Year3 - Year1)
  )
)

which gives

  ID Year1 Year2 Year3 Size1 Size2 Size3 Difference
1  A  2021  2022    NA    10    15    NA        5.0
2  B  2022  2023  2024    20    21    25        2.5
3  C  2021  2022    NA     5    20    NA       15.0