SORT_THEM

# https://www.codechef.com/problems/SORT_THEM
def sortedPos(ch):
    sortedd = 'abcdefghijklmnopqrstuvwxyz'
    return sortedd.find(ch)
#you can directly do char comparision, instead of sortedPos comparision
def checkIfSortable(s,p,lenn):
    partner = p[::-1]
    dp = [[float('inf')] * 2 for _ in range(lenn)]
    dp[0]= [0,1]
    #for a single char we dont need to care about prev ,just think what will be the result for single char s string
    for ind in range(1,lenn):
        prev = s[ind-1]
        curr = s[ind]
        '''
        you can use the partner with index where you find the s char in p
        partner is nothing but reversed p ,which satisfies x calculation for index i
        so if you want p(x) just take partner(i)
        '''
        changedPrev = partner[p.find(prev)]
        changedCurr = partner[p.find(curr)]
        if sortedPos(prev)<=sortedPos(curr):#cond1
            dp[ind][0] = dp[ind-1][0]
        if sortedPos(changedPrev)<=sortedPos(curr):
            dp[ind][0] = min(dp[ind][0],dp[ind-1][1])
            '''
            you are taking dp[ind][0] instead of dp[ind-1][0] for min operations, because you are not sure wether cond1 satisfied or not only dp[ind][0] will tell you at this point of time.
            '''


        if sortedPos(prev)<=sortedPos(changedCurr):
            dp[ind][1] = 1+dp[ind-1][0]
        if sortedPos(changedPrev)<=sortedPos(changedCurr):
            dp[ind][1] = min(dp[ind][1],1+dp[ind-1][1])

    res = min(dp[lenn-1][0],dp[lenn-1][1])
    return -1 if res==float('inf') else res

tc = int(input())
for _ in range(tc):
    lenn = int(input())
    s = input()
    p = input()

    print(checkIfSortable(s,p,lenn))