I want to have a user-defined function that will return the first integer of a subset of a vector so that the values of the subset are unique, increasing by 1, and of a detailed length.

For example, using the input data:

v <- c(3, 4, 5, 6, 15, 16, 25, 26, 27)

l <- 3

The feasible subset of the vector would be:

c(3, 4, 5)

c(4, 5, 6)

c(25, 26, 27)

and I try to get the function to choose one of these at random and return the first/lowest number, which is either 3, 4, or 25.