## https://yukicoder.me/problems/no/1924

def solve(S, index_array, p_set, p):
    has_p = False
    array = []
    prev = False
    for i in index_array:
        if S[i] == p:
            has_p = True
            prev = False
        else:
            if not prev:
                array.append([i])
                prev = True
            else:
                array[-1].append(i)            
    
    ans = 0
    if has_p:
        ans += 1

    for ar in array:
        min_ans = float("inf")
        for r in ("R", "G", "B"):
            if r not in p_set:
                p_set.add(r)
                a = solve(S, ar, p_set, r)
                p_set.remove(r)
                min_ans = min(min_ans, a)
        ans += min_ans
    return ans




def main():
    N = int(input())
    S = input()

    p_set = set()
    index_array = [i for i in range(N)]
    answer = float("inf")
    for p in ("R", "G", "B"):
        p_set.add(p)
        ans = solve(S, index_array, p_set, p)
        p_set.remove(p)
        answer = min(answer, ans)
    print(answer)
    




if __name__ == "__main__":
    main()