Problem: There are n left boots and n right boots. Each boot has a color which is denoted as a lowercase Latin letter or a question mark ('?'). Thus, you are given two strings l and r, both of length n. The character l_i stands for the color of the i-th left boot and the character r_i stands for the color of the i-th right boot.

A lowercase Latin letter denotes a specific color, but the question mark ('?') denotes an indefinite color. Two specific colors are compatible if they are exactly the same. An indefinite color is compatible with any (specific or indefinite) color.

For example, the following pairs of colors are compatible: ('f', 'f'), ('?', 'z'), ('a', '?') and ('?', '?'). The following pairs of colors are not compatible: ('f', 'g') and ('a', 'z').

Compute the maximum number of pairs of boots such that there is one left and one right boot in a pair and their colors are compatible.

Print the maximum number of such pairs and the pairs themselves. A boot can be part of at most one pair.

Input

The first line contains n (1 ≤ n ≤ 150000), denoting the number of boots for each leg (i.e. the number of left boots and the number of right boots).

The second line contains the string l of length n. It contains only lowercase Latin letters or question marks. The i-th character stands for the color of the i-th left boot.

The third line contains the string r of length n. It contains only lowercase Latin letters or question marks. The i-th character stands for the color of the i-th right boot.

Output

Print k — the maximum number of compatible left-right pairs of boots, i.e. pairs consisting of one left and one right boot which have compatible colors.

The following k lines should contain pairs a_j, b_j (1 ≤ a_j, b_j ≤ n). The j-th of these lines should contain the index a_j of the left boot in the j-th pair and index b_j of the right boot in the j-th pair. All the numbers a_j should be distinct (unique), all the numbers b_j should be distinct (unique).

If there are many optimal answers, print any of them.

Examples

Input


10
codeforces
dodivthree


Output


5
7 8
4 9
2 2
9 10
3 1


Input


7
abaca?b
zabbbcc


Output


5
6 5
2 3
4 6
7 4
1 2


Input


9
bambarbia
hellocode


Output


0


Input


10
code??????
??????test


Output


10
6 2
1 6
7 3
3 5
4 8
9 7
5 1
2 4
10 9
8 10

Solution (in JAVA):

Solution:
//    A Computer is Like a mischievous genie.
//    It will give you exactly what you ask for,
//    but not always what you want
//    A code by Rahul Verma


import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.SortedSet;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import java.util.TreeSet;
import javax.swing.plaf.basic.BasicScrollPaneUI.HSBChangeListener;


public class Main {


    static class Clock {

        protected long start, stop;

        public void start() {
            start = System.currentTimeMillis();
        }

        public void stop() {
            stop = System.currentTimeMillis();
        }

        public String getTime() {
            return ((stop - start) + " ms");
        }
    }


    public static PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));

    static class FastReader {

        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new
                InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String[] nextSArray() {
            String sr[] = null;
            try {
                sr = br.readLine().trim().split(" ");
            } catch (IOException e) {
                e.printStackTrace();
            }
            return sr;
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return str;
        }
    }


    static long powmodulo(long a, long p) {
        if (p == 0) {
            return 1 % mod;
        }
        if (p == 1) {
            return a % mod;
        }
        long ans = 1;
        while (p > 0) {
            if ((p & 1) > 0) {
                ans = (ans * a) % mod;
            }
            a = (a * a) % mod;
            p = p >> 1;
        }
        return ans % mod;
    }


    static long mod = 1000000007;

    static long gcd(long a, long b) {
        if (a == 0) {
            return b;
        }
        return gcd(b % a, a);
    }

    static long fast_powerNumbers(long a, long n) {
        if (n == 1) {
            return a;
        }
        long ans = fast_powerNumbers(a, n / 2);
        if (n % 2 == 0) {
            return (ans * ans);
        } else {
            return ((ans * ans) * (a));
        }
    }


    static void dfs_helper(int[][] arr, int i, int j, int team, int n, int m) {
        arr[i][j] = team;
        if (i - 1 >= 0 && arr[i - 1][j] == 1) {
            dfs(arr, i - 1, j, team, n, m);
        }
        if (j - 1 >= 0 && arr[i][j - 1] == 1) {
            dfs(arr, i, j - 1, team, n, m);
        }
        if (i + 1 < n && arr[i + 1][j] == 1) {
            dfs(arr, i + 1, j, team, n, m);
        }
        if (j + 1 < m && arr[i][j + 1] == 1) {
            dfs(arr, i, j + 1, team, n, m);
        }

    }

    static void dfs(int[][] arr, int i, int j, int team, int n, int m) {
        dfs_helper(arr, i, j, team, n, m);

    }

    static int parent[];
    static int rank[];

    static int find(int i) {
        if (parent[i] == -1) {
            parent[i] = i;
            return i;
        }

        if (parent[i] == i) {
            return i;
        } else {
            parent[i] = find(parent[i]);
        }
        return parent[i];

    }

    static void unite(int s1, int s2) {

        if (rank[s1] > rank[s2]) {
            parent[s2] = s1;
            rank[s1] += rank[s2];
        } else {
            parent[s1] = s2;
            rank[s2] += rank[s1];
        }
    }


    static int arr[];

    static void seive(int n) {
        arr = new int[n + 1];
        arr[0] = arr[1] = 1;
        for (int i = 4; i <= n; i = i + 2) {
            arr[i] = 1;
        }
        for (int i = 3; i * i <= n; i = i + 2) {
            if (arr[i] == 0) {
                for (int j = i * i; j <= n; j = j + i) {
                    arr[j] = 1;
                }

            }
        }
    }

    static int dfss(int i)
    {
        if(dp[i]!=-1)
            return dp[i];

        int maxd=0;
        for(Integer nbr:hm.get(i))
        {
           int d = 1+ dfss(nbr);
           maxd=Math.max(maxd,d);
        }
        max=Math.max(maxd,max);
        return dp[i]=maxd;

    }


   static int dp[];
    static int max=0;
    static HashMap<Integer,ArrayList<Integer>>hm;
    public static void main(String[] args) throws Exception {
        FastReader sc = new FastReader();

        Clock clock = new Clock();
        clock.start();

            int n=sc.nextInt();
            String l =sc.nextLine();
            String r =sc.nextLine();
            HashMap<Character,SortedSet<Integer>>hm=new HashMap<>();
            HashMap<Character,SortedSet<Integer>>hm2=new HashMap<>();
            for (int i = 0; i <n ; i++) {
                char ch=l.charAt(i);
                if(!hm.containsKey(ch))
                    hm.put(ch, new TreeSet<>());
                hm.get(ch).add(i);
            }

            for (int i = 0; i <n ; i++) {
                char ch=r.charAt(i);
                if(!hm2.containsKey(ch))
                    hm2.put(ch, new TreeSet<>());
                hm2.get(ch).add(i);
            }
            ArrayList<Pair>al=new ArrayList<>();
            for(Character ch: hm.keySet())
            {
                if(ch=='?')
                    continue;
                while(!hm.get(ch).isEmpty())
                {
                    if(hm2.containsKey(ch)&&!hm2.get(ch).isEmpty())
                    {
                        int a=hm.get(ch).first();
                        int b=hm2.get(ch).first();
                        al.add(new Pair(a+1,b+1));
                        hm.get(ch).remove(a);
                        hm2.get(ch).remove(b);
                    }
                    else if(hm2.containsKey('?')&&!hm2.get('?').isEmpty())
                    {
                        int a=hm.get(ch).first();
                        int b=hm2.get('?').first();
                        al.add(new Pair(a+1,b+1));
                        hm.get(ch).remove(a);
                        hm2.get('?').remove(b);
                    }
                    else
                        break;
                }


            }
            while(hm.containsKey('?')&&!hm.get('?').isEmpty())
            {
                int a =hm.get('?').first();
                hm.get('?').remove(a);
                for(Character ch:hm2.keySet())
                {
                    if(!hm2.get(ch).isEmpty())
                    {
                        int b=hm2.get(ch).first();
                        hm2.get(ch).remove(b);
                        al.add(new Pair(a+1,b+1));
                        break;
                    }
                }

            }
            out.println(al.size());
            for(Pair p: al)
                out.println(p.a+" "+p.b);
            out.close();

    }}



class Pair {

    int a;
    int b;

    Pair(int a, int b) {
        this.a = a;
        this.b = b;
    }
}
class DSU
{
    int parent[];
    int rank[];
    DSU(int n)
    {
        this.parent=new int[n];
        this.rank=new int[n];
        Arrays.fill(parent,-1);
        Arrays.fill(rank,1);
    }

    int find(int s1)
    {
        if(parent[s1]==-1)
            return s1;
        return parent[s1]=find(parent[s1]);
    }

    void unite(int s1,int s2)
    {
        int p1=this.find(s1);
        int p2=this.find(s2);
        if(p1==p2)
            return;
        if(rank[p1]>rank[p2])
        {
            parent[p2]=p1;
            rank[p1]+=rank[p2];
        }
        else
        {
            parent[p1]=p2;
            rank[p2]+=rank[p1];
        }
    }


}


class Graph {

    HashMap<Integer, ArrayList<Integer>> hm;

    Graph() {
        hm = new HashMap<>();
    }

    Graph(int n) {

        hm = new HashMap<>();
        for (int i = 0; i < n; i++) {
            hm.put(i, new ArrayList<Integer>());
        }
    }

    // function for adding an edge.................................................
    public void addEdge(int a, int b, boolean isDir) {
        if (isDir) {
            if (hm.containsKey(a)) {
                hm.get(a).add(b);
            } else {
                hm.put(a, new ArrayList<>(Arrays.asList(b)));
            }
        } else {
            if (hm.containsKey(a)) {
                hm.get(a).add(b);
            } else if (!hm.containsKey(a)) {
                hm.put(a, new ArrayList<>(Arrays.asList(b)));
            }

            if (hm.containsKey(b)) {
                hm.get(b).add(a);
            } else if (!hm.containsKey(b)) {
                hm.put(b, new ArrayList<>(Arrays.asList(a)));
            }
        }
    }


}

// out.println(al.toString().replaceAll("[\\[|\\]|,]",""));