倒着搞会方便一点

n=int(input())
s=input()
d=[0]*128
ans=0
for c in s[::-1]:
    x=ord(c)
    for i in range(97,123):
        if(i==x):
            d[i]+=1
        else:
            ans+=d[i]*(d[i]-1)//2
print(ans)