num_actions = int(input(""))

my_ammo = 0
my_score = 0

their_ammo = 0

for their_action in input(""):

    if their_action == "R":
        their_ammo += 1

    if their_action == "R" and my_ammo == 0:
        # print("R", end="")
        my_ammo += 1
    elif their_action == "R" and my_ammo > 0:
        # print("F", end="")
        my_ammo -= 1
        my_score += 1
    elif their_action == "B":
        # print("R", end="")
        my_ammo += 1
    elif their_action == "F" and their_ammo > 0:
        # print("B", end="")
        their_ammo -= 1
        pass
    elif their_action == "F" and their_ammo == 0:
        if my_ammo > 0:
            # print("F", end="")
            my_ammo -= 1
            my_score += 1
        else:
            # print("R", end="")
            my_ammo += 1

print(my_score)