#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;}