n = int(input()) if n < 3: print(0) else: s = input() dp = [{} for i in range(n)] # dp[i]记录i之后各个字母出现的次数 ans = 0 for i in range(n-2, -1, -1): dp[i] = dp[i + 1].copy() key = s[i + 1] val = dp[i+1].get(key, 0) + 1 dp[i][key] = val for i in range(n - 3, -1, -1): for key, val in dp[i].items(): # 和字母i不...