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:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class defaultx_t {
    public static void main(String[] args) {
        in = new FastReader();
        int n=ni();
        String str1=n();
        String str2=n();
        HashMap<Character,ArrayList<Integer>> hm=new HashMap<>();
        int pos=-1;
        for (char c:str1.toCharArray()){
            pos++;
            ArrayList<Integer> al=hm.getOrDefault(c,new ArrayList<>());
            al.add(pos);
            hm.put(c,al);
        }
        int ans=0;
        ArrayList<pair> build=new ArrayList<>();
        ArrayList<Integer> quesmarkindexsave=new ArrayList<>();
        pos=-1;
        for (char c:str2.toCharArray()){
            pos++;
            if (c=='?')quesmarkindexsave.add(pos);
            else {
                if (hm.containsKey(c)){
                    ArrayList<Integer> al=hm.get(c);
                    build.add(new pair(pos,al.get(al.size()-1)));
                    al.remove(al.size()-1);
                    if (al.size()==0){
                        hm.remove(c);
                    }
                    ans++;
                }else if (hm.containsKey('?')){
                    ArrayList<Integer> al=hm.get('?');
                    build.add(new pair(pos,al.get(al.size()-1)));
                    al.remove(al.size()-1);
                    if (al.size()==0){
                        hm.remove('?');
                    }
                    ans++;
                }
            }
        }
        ArrayList<Integer> baccha=new ArrayList<>();
        for (int i=0;i<26;i++){
            char q= (char) ('a'+i);
            ArrayList<Integer> al=hm.getOrDefault(q,new ArrayList<>());
            baccha.addAll(al);
        }
        baccha.addAll(hm.getOrDefault('?',new ArrayList<>()));
         pos=0;
        for (int i=0;i<quesmarkindexsave.size();i++){
            if (pos>=baccha.size())break;
            else {
                build.add(new pair(quesmarkindexsave.get(i),baccha.get(pos)));
                pos++;
                ans++;
            }
        }
        System.out.println(ans);
        StringBuilder sb=new StringBuilder();
        for (int i=0;i<build.size();i++){
            sb.append(build.get(i)).append("\n");
        }
        System.out.println(sb);


    }
    static long moves(long a,long b){
        if (a==b)return 0;
        if (a>b)return Integer.MAX_VALUE;
        long a1=a*2;
        long a2=a*3;
        long f1=moves(a1,b);
        long f2=moves(a2,b);
        return Math.min(f1,f2)+1;
    }
    static class pair{
        int x;
        int y;
        pair(int y,int x){
            this.x=x;
            this.y=y;
        }
        @Override
        public String toString(){
            return (this.x+1)+" "+(this.y+1);
        }
    }
    public static long binarySearch(long low, long high) {
        while (high - low > 1) {
            long mid = (high - low)/2 + low;
            //System.out.println(mid);
            if (works(mid)) {
                high = mid;
            } else {
                low = mid;
            }
        }
        return (works(low) ? low : high);
    }
    static long fast_exp_with_mod(long base, long exp) {
        long MOD=1000000000+7;
        long res=1;
        while(exp>0) {
            if(exp%2==1) res=(res*base)%MOD;
            base=(base*base)%MOD;
            exp/=2;
        }
        return res%MOD;
    }
    public static long gcd(long a, long b)
    {
        if (a == 0)
            return b;
        return gcd(b%a, a);
    }
    static class my_no{
        long num;
        long denom;

        @Override
        public String toString() {
            if (denom<0){
                this.num=-this.num;
                this.denom=-this.denom;
            }
            if (num==0)return "0";
            return (num+"/"+denom);
        }

        my_no(int no){
            this.num=no;
            this.denom=1;
        }
        my_no(long num,long denom){
            this.num=num;
            this.denom=denom;
        }

        my_no multiply(my_no obj){
            long num1=obj.num;
            long denom1=obj.denom;
            long n=num1*num;
            long d=denom1*denom;
            long gcd=gcd(n,d);
            n/=gcd;
            d/=gcd;
            return new my_no(n,d);

        }
//        my_no multiply(my_no obj){
//            long num1=obj.num;
//            long denom1=obj.denom;
//            long num2=this.num;
//            long denom2=this.denom;
//
//        }

        my_no multiply(int no){
            long n=num*no;
            long d=denom;
            long gcd=gcd(n,d);
            n/=gcd;
            d/=gcd;
            return new my_no(n,d);

        }
    }
    static void memset(int[][] arr,int val){
        for (int i=0;i<arr.length;i++){
            for (int j=0;j<arr[i].length;j++){
                arr[i][j]=val;
            }
        }
    }
    static void memset(int[] arr,int val){
        for (int i=0;i<arr.length;i++){
            arr[i]=val;
        }
    }


    static void memset(long[][] arr,long val){
        for (int i=0;i<arr.length;i++){
            for (int j=0;j<arr[i].length;j++){
                arr[i][j]=val;
            }
        }
    }
    static void memset(long[] arr,long val){
        for (int i=0;i<arr.length;i++){
            arr[i]=val;
        }
    }
    static private boolean works(long test){
        return true;
    }


    static void reverse(char[] arr ,int i,int j){
        if (i==j)
            return;
        while (i<j){
            char temp=arr[i];
            arr[i]=arr[j];
            arr[j]=temp;
            ++i;
            --j;
        }
    }
    static int[]  takeIntegerArrayInput(int no){
        int[] arr=new int[no];
        for (int i=0;i<no;++i){
            arr[i]=ni();
        }
        return arr;
    }
    static long fast_Multiply(long no , long pow){
        long result=1;
        while (pow>0){
            if ((pow&1)==1){
                result=result*no;
            }
            no=no*no;
            pow>>=1;
        }
        return result;
    }

    static long[]  takeLongArrayInput(int no){
        long[] arr=new long[no];
        for (int i=0;i<no;++i){
            arr[i]=ni();
        }
        return arr;
    }
    static final long MOD = (long)1e9+7;
    static FastReader in;


    static void p(Object o){
        System.out.print(o);
    }

    static void pn(Object o){
        System.out.println(o);
    }

    static String n(){
        return in.next();
    }

    static String nln(){
        return in.nextLine();
    }

    static int ni(){
        return Integer.parseInt(in.next());
    }

    static int[] ia(int N){
        int[] a = new int[N];
        for(int i = 0; i<N; i++)a[i] = ni();
        return a;
    }

    static long[] la(int N){
        long[] a = new long[N];
        for(int i = 0; i<N; i++)a[i] = nl();
        return a;
    }

    static long nl(){
        return Long.parseLong(in.next());
    }

    static double nd(){
        return Double.parseDouble(in.next());
    }

    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();
        }

        String nextLine(){
            String str = "";
            try{
                str = br.readLine();
            }catch (IOException e){
                e.printStackTrace();
            }
            return str;
        }
    }
    static void println(String[] arr){
        for (int i=0;i<arr.length;++i){
            System.out.println(arr[i]);
        }
    }

}