1215_C. Swap Letters
Monocarp has got two strings s and t having equal length. Both strings consist of lowercase Latin letters "a" and "b". 

Monocarp wants to make these two strings s and t equal to each other. He can do the following operation any number of times: choose an index pos_1 in the string s, choose an index pos_2 in the string t, and swap s_{pos_1} with t_{pos_2}.

You have to determine the minimum number of operations Monocarp has to perform to make s and t equal, and print any optimal sequence of operations — or say that it is impossible to make these strings equal.

Input

The first line contains one integer n (1 ≤ n ≤ 2 ⋅ 10^{5}) — the length of s and t.

The second line contains one string s consisting of n characters "a" and "b". 

The third line contains one string t consisting of n characters "a" and "b". 

Output

If it is impossible to make these strings equal, print -1.

Otherwise, in the first line print k — the minimum number of operations required to make the strings equal. In each of the next k lines print two integers — the index in the string s and the index in the string t that should be used in the corresponding swap operation. 

Examples

Input


4
abab
aabb


Output


2
3 3
3 2


Input


1
a
b


Output


-1


Input


8
babbaabb
abababaa


Output


3
2 6
1 3
7 8

Note

In the first example two operations are enough. For example, you can swap the third letter in s with the third letter in t. Then s =  "abbb", t =  "aaab". Then swap the third letter in s and the second letter in t. Then both s and t are equal to "abab".

In the second example it's impossible to make two strings equal.
```python
a=int(input())
s=input()
r=input()
cmap=[]
tmap=[]
for i in range(len(s)):
    if(s[i]=='a' and r[i]=='b'):
        cmap.append(i)
        
    elif(s[i]=='b' and r[i]=='a'):
        tmap.append(i)
r=len(cmap)
t=len(tmap)
if(((r%2==1 and t%2==0) or (r%2==0 and t%2==1))):
    print(-1)
else:
    total=r//2+t//2
    if(r%2==1 and t%2==1):
        print(total+2)
        for i in range(0,len(cmap)-1,2):
            print(cmap[i]+1,cmap[i+1]+1)
        for i in range(0,len(tmap)-1,2):
            print(tmap[i]+1,tmap[i+1]+1)
        print(cmap[-1]+1,cmap[-1]+1)
        print(cmap[-1]+1,tmap[-1]+1)
    else:
        print(total)
        for i in range(0,len(cmap),2):
            print(cmap[i]+1,cmap[i+1]+1)
        for i in range(0,len(tmap),2):
            print(tmap[i]+1,tmap[i+1]+1)
            
    

```