Scanf.scanf "%d %s" (fun n s ->
    let rec loop_i i acc =
        let rec loop_j j acc =
            if j >= n then acc else
                let acc = if s.[j] = 'G' && s.[(i + j) / 2] = 'M' then acc + 1 else acc in
                loop_j (j + 2) acc
        in
        if i = n then acc else
            let acc = if s.[i] = 'U' then loop_j (i + 2) acc
                                     else acc
            in
            loop_i (i + 1) acc
    in
    loop_i 0 0 |> Printf.printf "%d\n"
)