def func(s,n,a):
    c=chr(a)
    if(n==1):
        if(s[0]==c):
            return 0
        return 1
    c1=0
    c2=0
    for i in range(n//2):
            if s[i]==c:
                c1+=1
    for i in range(n//2,n):
            if s[i]==c:
                c2+=1
    return min(n//2-c1+func(s[n//2:],n//2,a+1),n//2-c2+func(s[:n//2],n//2,a+1))
for _ in range(int(input())):
    n=int(input())
    s=input()
    print(func(s,n,97))