#include"bits/stdc++.h"
using namespace std;
#define FAST_IO ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); cout.setf(ios::fixed); cout.precision(20)
#define ll long long
//       Author
//        ::
//    (  NOBODY  )
//   No Copyright .....You can copy and paste it without my permission...................................
// !!!!!!!!!!!!!!!
#define pb push_back
#define N 300008
int a[N],b[N],c[N];
vector<pair<int,int>>vp;
int main()
{
    FAST_IO;
    int n;
    cin>>n;
    string s,t;
    cin>>s>>t;
    vector<int>v[30];
    for(int i=0;i<n;i++)
    {
       if(t[i]=='?')v[27].push_back(i);
       else v[t[i]-'a'].push_back(i);
        if(s[i]=='?')v[28].push_back(i);
    }
    for(int i=0;i<n;i++)
    {
        if(s[i]!='?')
        {
            int x=s[i]-'a';

            if(v[x].size()>0)
            {
                 int y=v[x].front();
                v[x].erase(v[x].begin(),v[x].begin()+1);
                vp.push_back(make_pair(i,y));
            }
            else if(v[27].size()>0)
            {
                int y=v[27].front();
                v[27].erase(v[27].begin(),v[27].begin()+1);
                vp.pb({i,y});
            }

        }
    }
    bool boo=true;
    while(v[28].size()>0&&boo==true)
    {
        int x=v[28].front();
        for(int i=0;i<27;i++)
        {
            boo=false;
            if(v[i].size()>0)
            {
                boo=true;
                int y=v[i].front();
                v[i].erase(v[i].begin(),v[i].begin()+1);
                v[28].erase(v[28].begin(),v[28].begin()+1);
                vp.pb({x,y});
                break;
            }
        }
    }
    while(v[27].size()>0&&v[28].size()>0)
    {
        int x=v[27].front();
        int y=v[28].front();
        v[27].erase(v[27].begin(),v[27].begin()+1);
         v[28].erase(v[28].begin(),v[28].begin()+1);
         vp.push_back(make_pair(y,x));
    }
    cout<<vp.size()<<endl;
    for(auto c:vp)
    {
        cout<<c.first+1<<" "<<c.second+1<<endl;
    }
    return 0;
}