merge multiple polygon in one bordered polygon

I am trying to merge two polygons (departaments) of Colombia in a single polygon. My goal is to keep only the edges of both polygons but this does not happen. Below I attach the code. Thanks for any help.

download.file("https://github.com/nebulae-co/colmaps/raw/master/data/departamentos.rda","departamentos")  load("departamentos")    library(sf)  library(dplyr)  library(rgdal)  library(mapview)    df1 = subset(x = departamentos, depto == "Bogotá, D. C.")  df2 = subset(x = departamentos, depto == "Cundinamarca")    df1@data$id <- df2@data$id <- "a"    df3 = rbind(df1, df2, makeUniqueIDs = TRUE)  plot(df3)  

Plot the two polygons

## option 1  df_12 <- sf::st_as_sf(df3) %>%    group_by(id) %>%    summarise(geometry = sf::st_union(geometry)) %>%    ungroup()    plot(as_Spatial(df_12))    ## option 2  plot(raster::aggregate(df3))    ## option 3  plot(rgeos::gUnaryUnion(df3))    ## optiom 4  plot(st_union(st_make_valid(st_as_sf(df3))))    ## option 5  plot(st_as_sf(rgeos::gBuffer(as_Spatial(st_make_valid(st_as_sf(df3))), byid=F, width=0)))  

My expected output

https://stackoverflow.com/questions/66739814/merge-multiple-polygon-in-one-bordered-polygon March 22, 2021 at 11:04AM