let k = 2
let n = 5

let rec f (n, k) =
    match (n, k) with
        |(0, _) -> 0
        |(1, 1) -> 1
        |(_, 0) -> 1
        |(_, _) -> f(n - 1, k - 1) + f(n - 1, k)
let rez = f (n, k)