#include<bits/stdc++.h>
using namespace std;

typedef long long int ll;
#define IOS ios_base::sync_with_stdio(0);  cin.tie(0); cout.tie(0);

typedef pair<int,int>pr;
#define all(i)     i.begin() , i.end()
#define ft     first
#define sn     second
#define pb push_back

const int mod = 1e9+7;
#define en "\n"
#define dbg cout<<"rony\n";

vector<int>g[30];
int vis[30];
#define MAXN 200010
vector<pair<int,int>>v;

void dfs(int nd)
{
  vis[nd] = 1;
  for(auto i:g[nd])
  {
     if(vis[i] == 1) continue;
     v.pb({i,nd});
     dfs(i);
  }
}
void solve()
{
   int n;
   cin >> n;
   string s,t;
   cin >> s >> t;
   map<pair<char,char>,int>ma;

   for(int i = 0;i < n;i++)
   {
    if(s[i] == t[i]) continue;
    char c = max(s[i],t[i]);
    char d = min(s[i],t[i]);
    if(ma[{d,c}] == 1) continue;

    ma[{d,c}]++;
     g[s[i]-'a'].pb(t[i]-'a');
     g[t[i]-'a'].pb(s[i] - 'a');
   }

   for(int i = 0;i < 26;i++)
   {
     if(vis[i] == 0 && g[i].size() > 0)
     {
       dfs(i);
     }
   }

   cout<<v.size()<<en;
   for(auto i:v)
   {
    char c = i.ft + 'a';
    char d = i.sn + 'a';
     cout<<c<<" "<<d<<en;
   }
} 

int main()
{
    IOS;
    int t;
   //cin >> t;
   t = 1;
    while ( t-- )
    {

        solve();
    }
    return 0;
}