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#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <cstring>
#include <vector>
#include <cstdio>
#include <map>
#include <utility>
#include <queue>
#include <stack>
#include <deque>
#include <iomanip>
#include <set>
#include <fstream>
#include <ctime>
#include <cstdlib>

#define INF (1<<30)
#define F first
#define S second
#define pb push_back
#define mp make_pair
#define ll long long
#define ld long double
#define lld long long double
#define pii pair<ll,ll>
#define mod 1000000007
#define endl '\n'
#define BYKKKSGIUT

// меняй все переменные

#define TASKNAME ""

using namespace std;

char used['z'+100];

int main(){
    // freopen(TASKNAME".in", "r", stdin);
    // freopen(TASKNAME".out", "w", stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    string s1,s2;
    cin >> s1;
    cin >> s2;
    int n = s1.length();
    int cnt = 0;
    for(int i = 0; i < n; i++){
        if(s1[i]!=s2[i]){
            if((used[s1[i]]!=0 && used[s1[i]]!=s2[i]) || (used[s2[i]]!=0 && used[s2[i]]!=s1[i])){
                cout << -1 << endl;
                return 0;
            }else{
                if(used[s1[i]]==0 && used[s2[i]] == 0){
                    // cout << s1[i] << ' ' << s2[i] << endl;
                    cnt++;
                }
                used[s1[i]] = s2[i];
            }
        }
    }
    cout << cnt << endl;
    set<char> s;
    for(char i = 'a'; i <= 'z'; i++){
        if(used[i]!=0){
            s.insert(used[i]);
            if(s.find(i) == s.end())
                cout << i << ' ' << used[i] << endl;
        }
    }
    return 0;
}