from collections import*
n=int(input())
s=input()
dp=defaultdict(int)
dp['']=1
for i in range(1,n+1):
  c=s[i-1]
  for k,v in dp.copy().items():
    if k and len(k)<5:
      if len(k)==2:
        if k[0]==c:
          dp[k+c]+=v
      else:
        if c not in k:
          dp[k+c]+=v
    elif k=='':
      dp[c]+=v
  # print(i,dp)
ans=0
for k,v in dp.items():
  if len(k)==5:
    ans+=v
print(ans)