#pragma comment(linker, "/STACK:102400000, 102400000")

#include<cstdio>

#include<cstring>

#include<cstdlib>

#include<cctype>

#include<cmath>

#include<iostream>

#include<sstream>

#include<iterator>

#include<algorithm>

#include<string>

#include<vector>

#include<set>

#include<map>

#include<stack>

#include<deque>

#include<queue>

#include<list>

#define Min(a, b) ((a < b) ? a : b)

#define Max(a, b) ((a < b) ? b : a)

const double eps = 1e-8;

inline int dcmp(double a, double b){

    if(fabs(a - b) < eps) return 0;

    return a > b ? 1 : -1;

}

typedef long long LL;

typedef unsigned long long ULL;

const int INT_INF = 0x3f3f3f3f;

const int INT_M_INF = 0x7f7f7f7f;

const LL LL_INF = 0x3f3f3f3f3f3f3f3f;

const LL LL_M_INF = 0x7f7f7f7f7f7f7f7f;

const int dr[] = {0, 0, -1, 1, -1, -1, 1, 1};

const int dc[] = {-1, 1, 0, 0, -1, 1, -1, 1};

const int MOD = 1e9 + 7;

const double pi = acos(-1.0);

const int MAXN = 1000 + 10;

const int MAXT = 10000 + 10;

using namespace std;

char a[MAXN], b[MAXN];

const string s = "abcdefghijklmnopqrstuvwxyz";

map<char, int> mp;

int vis[30];

int ans[30];

void init(){

    for(int i = 0; i < 26; ++i){

        mp[s[i]] = i + 1;

    }

}

int main(){

    init();

    while(scanf("%s%s", a, b) == 2){

        memset(vis, 0, sizeof vis);

        memset(ans, 0, sizeof ans);

        int len = strlen(a);

        int cnt = 0;

        bool ok = true;

        for(int i = 0; i < len; ++i){

            int tmpa = mp[a[i]], tmpb = mp[b[i]];

            if(!vis[tmpa] && !vis[tmpb]){

                vis[tmpa] = tmpb;

                vis[tmpb] = tmpa;

                if(tmpa != tmpb){

                    ans[cnt++] = tmpa;

                }

            }

            else if(vis[tmpa] != tmpb || vis[tmpb] != tmpa){

                ok = false;

                break;

            }

        }

        if(!ok){

            printf("-1\n");

        }

        else{

            printf("%d\n", cnt);

            for(int i = 0; i < cnt; ++i){

                printf("%c %c\n", ans[i] + 'a' - 1, vis[ans[i]] + 'a' - 1);

            }

        }

    }

    return 0;

}