Condition and row and column with a double loop

I get a problem about "subscript out of bounds", what I want is to get the first and last month for each observation where I have three consecutive "1" or "True". I want to create 2 new column "begin" and "end" where I get repectively the first month and the last month. In my example for the first observation : begin equal to avril and end equal to juin In the 5 observation : begin equal to fevrier and end equal to avril In the 9 observation : begin equal to janvier and end equal to mars ...

I tried to do this :

nom <- letters[1:5]
pseudo <- paste(name, 21:25, sep = "")
janvier <- c(0, 1, 1, 1, 0)
fevrier <- c(1, 1, 1, 1, 1)
mars <- c(0, 0, 0, 1, 1)
avril <- c(1, 1, 1, 0, 1)
mai <- c(1, 0, 1, 1, 1)
juin <- c(1, 1, 0, 1, 0)

df <- data.frame(nom =nom, pseudo = pseudo, janvier = janvier,
                 fevrier = fevrier, mars = mars, avril = avril,
                 mai = mai, juin = juin)

dfm <- as.matrix(df[, -c(1, 2)])

my_matrix <- matrix(nrow = 10, ncol = 6)


for(i in 1:dim(dfm)[1]){
  for(j in 1:dim(dfm)[2]){
    if(dfm[i, j] + dfm[i, j+1] + dfm[i, j+2] == 3){
      my_matrix[i, j] <- "periode_ok"
      my_matrix[i, j+1] <- "periode_ok"
      my_matrix[i, j+2] <- "periode_ok"
    } 
  }
}

The ouput should be this :

begin <- c("avril", "no  info", "no info",
           "janvier", "fevrier", "avril", "no info",
           "no info", "janvier", "fevrier")
end <- c("juin", "no info", "no info", "mars",
         "avril", "juin", "no info", "no info",
         "mars", "avril")

output <- data.frame(nom =nom, pseudo = pseudo, janvier = janvier,
                 fevrier = fevrier, mars = mars, avril = avril,
                 mai = mai, juin = juin, begin = begin,end = end)

Any help will be apreciated