Calculating Session Duration in R

I have a dataset with a session id, user id, TimeStamp in UNIX (which I converted using lubridate), and the converted TimeStamp column.

I would like to add another column called session_duration (in seconds) which is the difference between the max_time and min_time grouped by Session and User. For instance, for session # 123 and user 345, the session duration is [14-06-2022 17:44:37] - [14-06-2022 17:44:32] which is 5 seconds.

This is what my current code looks like. The timestamp has successfully converted, but I am facing an issue with the session duration column.

library(tidyverse)
library(lubridate)
df <- df %>%
  mutate(timestamp = as_datetime(ts_unix/1000)) %>%
  group_by (session, user, timestamp) %>%
  mutate(session_duration = difftime (max(timestamp), min(timestamp), units = "secs"))

Can someone please help me figure out the session_duration column? Thank you.

library(tidyverse)
library(lubridate)

df %>% 
  group_by(Session, User) %>% 
  mutate(session_duration = max(TimeStamp) - min(TimeStamp))

# A tibble: 11 × 5
# Groups:   Session, User [3]
   Session  User ts_UNIX        TimeStamp           session_duration
     <dbl> <dbl> <chr>          <dttm>              <drtn>          
 1     123   345 UNIX Timestamp 2022-06-14 17:44:32    5 secs       
 2     123   345 UNIX Timestamp 2022-06-14 17:44:33    5 secs       
 3     123   345 UNIX Timestamp 2022-06-14 17:44:37    5 secs       
 4     124   346 UNIX Timestamp 2022-06-14 15:50:10 2390 secs       
 5     124   346 UNIX Timestamp 2022-06-14 15:51:01 2390 secs       
 6     124   346 UNIX Timestamp 2022-06-14 16:30:00 2390 secs       
 7     125   345 UNIX Timestamp 2022-06-14 23:55:30  329 secs       
 8     125   345 UNIX Timestamp 2022-06-14 23:58:50  329 secs       
 9     125   345 UNIX Timestamp 2022-06-14 23:59:45  329 secs       
10     125   345 UNIX Timestamp 2022-06-15 00:00:32  329 secs       
11     125   345 UNIX Timestamp 2022-06-15 00:00:59  329 secs