def count(s,c):
    ct=0
    for i in s:
        if i==c:
            ct+=1
    return ct
def func(s,n,a):
    c=chr(a)
    if(n==1):
        if(s[0]==c):
            return 0
        return 1
    return min(n//2-count(s[:n//2],c)+func(s[n//2:],n//2,a+1),n//2-count(s[n//2:],c)+func(s[:n//2],n//2,a+1))
for _ in range(int(input())):
    n=int(input())
    s=input()
    print(func(s,n,97))