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

import itertools

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

    min_answer = float("inf")
    for p in itertools.permutations(["R", "G", "B"]):
        # 一番最初は全部を同じ色で塗りつぶす
        answer = 1
        array = []
        prev = False
        for i in range(N):
            if S[i] != p[0]:
                if len(array) == 0:
                    array.append([i])
                    prev = True
                else:
                    if not prev:
                        array.append([i])
                        prev = True
                    else:
                        array[-1].append(i)
            else:
                prev = False
        
        answer += len(array)
        new_array = []
        for ar in array:
            prev = False
            for j in ar:
                if S[j] != p[1]:
                    if len(array) == 0:
                        new_array.append([j])
                        prev = True
                    else:
                        if not prev:
                            new_array.append([j])
                            prev = True
                        else:
                            new_array[-1].append(j)
                else:
                    prev = False
        answer += len(new_array)
        min_answer = min(min_answer, answer)
    print(min_answer)






if __name__ == "__main__":
    main()