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.*;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.*;
public class Main {

    public void sort(long b[]){
        Random rd = new Random();
        for(int i=1;i<b.length;++i){
            int c = rd.nextInt(i);
            long v = b[c];
            b[c] = b[i];
            b[i] = v;
        }
        Arrays.sort(b);
    }
    public void sort(int b[]){
        Random rd = new Random();
        for(int i = 1;i<b.length;++i){
            int c = rd.nextInt(i);
            int v = b[c];
            b[c]= b[i];
            b[i] = v;
        }
        Arrays.sort(b);
    }

    static int groups = 0;
    static int[] fa;
    static int[] sz;

    static void init(int n) {
        groups = n;
        fa = new int[n];
        for (int i = 1; i < n; ++i) {
            fa[i] = i;
        }
        sz = new int[n];
        Arrays.fill(sz, 1);
    }

    static int root(int p) {
        while (p != fa[p]) {
            fa[p] = fa[fa[p]];
            p = fa[p];
        }
        return p;
    }

    static void combine(int p, int q) {
        int i = root(p);
        int j = root(q);
        if (i == j) {
            return;
        }
        if (sz[i] < sz[j]) {
            fa[i] = j;
            sz[j] += sz[i];
        } else {
            fa[j] = i;
            sz[i] += sz[j];
        }
        groups--;
    }

    class Pair {
        long x;long y;
        Pair(long xx,long yy){
            x = xx;y= yy;
            long g = gcd(Math.abs(x),Math.abs(y));
            if(xx*yy>=0){
                x = Math.abs(x)/g;
                y = Math.abs(y)/g;
            }else{
                x = -Math.abs(x)/g;
                y = Math.abs(y)/g;
            }
        }

        public boolean equals(Object fk){
            Pair p = (Pair)fk;
            return x == p.x && y== p.y;
        }

        public int hashCode(){
            return (int)(x*37+y);
        }

    }

    void dfs(int g, List<Integer> go[],boolean vis[]){
        vis[g] = true;
        for(int u: go[g]){
            int to = u;
            if(!vis[to]) {
                dfs(to, go, vis);
            }
        }

    }

    public static void main(String[] args) throws Exception {
        new Main().run();}

        void solve(){
            int n  =ni();
            String a= ns();
            String b= ns();

            List<Integer> g[] = new ArrayList[27];
            for(int i=0;i<27;++i){
                g[i] = new ArrayList<>();
            }
            for(int i=0;i<n;++i){
                if(b.charAt(i)!='?') {
                    g[b.charAt(i) - 'a'].add(i + 1);
                }else{
                    g[26].add(i+1);
                }
            }
            int p[] = new int[27];
            List<Integer> fk = new ArrayList<>();
            List<int[]> res =new ArrayList<>();
            for(int i=0;i<n;++i){
                char c = a.charAt(i);
                if(c=='?'){
                    fk.add(i+1);continue;
                }
                if(p[c-'a']<g[c-'a'].size()){
                    res.add(new int[]{i+1,g[c-'a'].get(p[c-'a']++)});
                }else if(p[26]<g[26].size()){
                    res.add(new int[]{i+1,g[26].get(p[26]++)});
                }
            }
            int gk = 0;
            for(int i=0;i<27&&gk<fk.size();++i){
                while(p[i]<g[i].size()&&gk<fk.size()){
                    res.add(new int[]{fk.get(gk++), g[i].get(p[i]++)});
                }
            }

            println(res.size());
            for(int u[]:res){
                println(u[0]+" "+u[1]);
            }



















//         PriorityQueue<int[]> q= new PriorityQueue<>((x,y)->{return x[0]-y[0];});
//
//         q.offer(new int[]{0,0});
//         boolean inq[] = new boolean[n];
//         while(q.size()>0){
//             int c[]= q.poll();
//             for(int i=h[c[1]];i!=-1;i=ne[i]){
//                 if(!inq[to[i]]) {
//                     q.offer(new int[]{ wt[i], to[i]});
//                     inq[to[i]] = true;
//                 }
//             }
//         }





        }


       int err = 0;



    void go(int rt,int pt,int fa) {
        stk[rt] = pt;
        for(int i=h[rt];i!=-1;i=ne[i]) {
            if(to[i]==pt||to[i]==fa) continue;
            go(to[i],pt,rt);
        }
    }



    int ct = 0;
    void add(int u,int v,int w,int q){
        to[ct] = v;
        wt[ct] = w;
        qt[ct] = q;
        ne[ct] = h[u];
        h[u] = ct++;
//
//        to1[ct] = u;
//        wt1[ct] = w;
//        ne1[ct] = h1[v];
//        h1[v] = ct++;
    }
    int []h,ne,to,wt,qt,stk;


    long inv(long a, long MOD) {
        //return fpow(a, MOD-2, MOD);
        return a==1?1:(long )(MOD-MOD/a)*inv(MOD%a, MOD)%MOD;
    }


    void inverse(){
        int MAXN = 10000;
        long inv[] = new long[MAXN+1];
        inv[1] = 1; //注意初始化
        long mod = 1000000007;
        for (int i=2; i<=MAXN; ++i) {
            inv[i] = (mod - mod / i) * inv[(int) (mod % i)] % mod;
        }
    }



    // 计算某个特别大的组合数
    long C(long n,long m, long MOD) {
        if(m+m>n)m=n-m;
        long up=1,down=1;
        for(long i=0;i<m;i++)
        {
            up=up*(n-i)%MOD;
            down=down*(i+1)%MOD;
        }
        return up*inv(down, MOD)%MOD;
    }


    void solve2() {

            int n = ni();
            String s=  ns();
            int g[][] = new int[3][3];
            for(int i = 0;i<n;++i){
                char c = s.charAt(i);
                int idx = 0;
                if(c=='G'){
                    idx =1;
                }else if(c=='B'){
                    idx =2;
                }
                g[c][i%3]++;
            }







        }































//    int []h,to,ne,wt;

    long t1[];
    //    long t2[];
    void update(long[] t,int i,long v){
        for(;i<t.length;i+=(i&-i)){
            t[i] += v;
        }
    }
    long get(long[] t,int i){
        long s = 0;
        for(;i>0;i-=(i&-i)){
            s += t[i];
        }
        return s;
    }

    int equal_bigger(long t[],long v){
        int s=0,p=0;
        for(int i=Integer.numberOfTrailingZeros(Integer.highestOneBit(t.length));i>=0;--i) {
            if(p+(1<<i)< t.length && s + t[p+(1<<i)] < v){
                v -= t[p+(1<<i)];
                p |= 1<<i;
            }
        }
        return p+1;
    }


    long gcd(long a,long b){ return b==0?a: gcd(b,a%b);}
    InputStream is;PrintWriter out;
    void run() throws Exception {is = System.in;
      //  is = new FileInputStream("C:\\Users\\Luqi\\Downloads\\P1600_2.in");

        out = new PrintWriter(System.out);solve();out.flush();}
    private byte[] inbuf = new byte[2];
    public int lenbuf = 0, ptrbuf = 0;
    private int readByte() {
        if (lenbuf == -1) throw new InputMismatchException();
        if (ptrbuf >= lenbuf) {
            ptrbuf = 0;
            try {lenbuf = is.read(inbuf);} catch (IOException e) {throw new InputMismatchException();}
            if (lenbuf <= 0) return -1;
        }
        return inbuf[ptrbuf++];}
    private boolean isSpaceChar(int c) {return !(c >= 33 && c <= 126);}
    private int skip() {int b;while((b = readByte()) != -1 && isSpaceChar(b));return b;}
    private double nd() {return Double.parseDouble(ns());}
    private char nc() {return (char) skip();}
    private char ncc() {int b;while((b = readByte()) != -1 && !(b >= 32 && b <= 126));return (char)b;}
    private String ns() {int b = skip();StringBuilder sb = new StringBuilder();
        while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
            sb.appendCodePoint(b);b = readByte(); }
        return sb.toString();}
    private char[] ns(int n) {char[] buf = new char[n];int b = skip(), p = 0;
        while (p < n && !(isSpaceChar(b))) { buf[p++] = (char) b;b = readByte(); }
        return n == p ? buf : Arrays.copyOf(buf, p);}
    private String nline() {int b = skip();StringBuilder sb = new StringBuilder();
        while (!isSpaceChar(b) || b == ' ') { sb.appendCodePoint(b);b = readByte(); }
        return sb.toString();}
    private char[][] nm(int n, int m) {char[][] a = new char[n][];for (int i = 0; i < n; i++) a[i] = ns(m);return a;}
    private int[] na(int n) {int[] a = new int[n];for (int i = 0; i < n; i++) a[i] = ni();return a;}
    private long[] nal(int n) { long[] a = new long[n];for (int i = 0; i < n; i++) a[i] = nl();return a;}
    private int ni() { int num = 0, b; boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')){};
        if (b == '-') { minus = true; b = readByte(); }
        while (true) {
            if (b >= '0' && b <= '9') num = (num << 3) + (num << 1) + (b - '0');
            else return minus ? -num : num;
            b = readByte();}}
    private long nl() { long num = 0; int b;
        boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')){};
        if (b == '-') { minus = true; b = readByte(); }
        while (true) {
            if (b >= '0' && b <= '9')  num = num * 10 + (b - '0');
            else return minus ? -num : num;
            b = readByte();}}
    void print(Object obj){out.print(obj);}
    void println(Object obj){out.println(obj);}
    void println(){out.println();}
}