#include <bits/stdc++.h>
using namespace std;
struct AdjListNode {
  int dest;
  int weight;
  struct AdjListNode* next;
};
struct AdjList {
  struct AdjListNode* head;
};
struct Graph {
  int V;
  struct AdjList* array;
};
struct AdjListNode* newAdjListNode(int dest, int weight) {
  struct AdjListNode* newNode =
      (struct AdjListNode*)malloc(sizeof(struct AdjListNode));
  newNode->dest = dest;
  newNode->weight = weight;
  newNode->next = NULL;
  return newNode;
}
struct Graph* createGraph(int V) {
  struct Graph* graph = (struct Graph*)malloc(sizeof(struct Graph));
  graph->V = V;
  graph->array = (struct AdjList*)malloc(V * sizeof(struct AdjList));
  for (int i = 0; i < V; ++i) graph->array[i].head = NULL;
  return graph;
}
void addEdge(struct Graph* graph, int src, int dest, int weight) {
  struct AdjListNode* newNode = newAdjListNode(dest, weight);
  newNode->next = graph->array[src].head;
  graph->array[src].head = newNode;
  newNode = newAdjListNode(src, weight);
  newNode->next = graph->array[dest].head;
  graph->array[dest].head = newNode;
}
struct MinHeapNode {
  int v;
  int key;
};
struct MinHeap {
  int size;
  int capacity;
  int* pos;
  struct MinHeapNode** array;
};
struct MinHeapNode* newMinHeapNode(int v, int key) {
  struct MinHeapNode* minHeapNode =
      (struct MinHeapNode*)malloc(sizeof(struct MinHeapNode));
  minHeapNode->v = v;
  minHeapNode->key = key;
  return minHeapNode;
}
struct MinHeap* createMinHeap(int capacity) {
  struct MinHeap* minHeap = (struct MinHeap*)malloc(sizeof(struct MinHeap));
  minHeap->pos = (int*)malloc(capacity * sizeof(int));
  minHeap->size = 0;
  minHeap->capacity = capacity;
  minHeap->array =
      (struct MinHeapNode**)malloc(capacity * sizeof(struct MinHeapNode*));
  return minHeap;
}
void swapMinHeapNode(struct MinHeapNode** a, struct MinHeapNode** b) {
  struct MinHeapNode* t = *a;
  *a = *b;
  *b = t;
}
void minHeapify(struct MinHeap* minHeap, int idx) {
  int smallest, left, right;
  smallest = idx;
  left = 2 * idx + 1;
  right = 2 * idx + 2;
  if (left < minHeap->size &&
      minHeap->array[left]->key < minHeap->array[smallest]->key)
    smallest = left;
  if (right < minHeap->size &&
      minHeap->array[right]->key < minHeap->array[smallest]->key)
    smallest = right;
  if (smallest != idx) {
    MinHeapNode* smallestNode = minHeap->array[smallest];
    MinHeapNode* idxNode = minHeap->array[idx];
    minHeap->pos[smallestNode->v] = idx;
    minHeap->pos[idxNode->v] = smallest;
    swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
    minHeapify(minHeap, smallest);
  }
}
int isEmpty(struct MinHeap* minHeap) { return minHeap->size == 0; }
struct MinHeapNode* extractMin(struct MinHeap* minHeap) {
  if (isEmpty(minHeap)) return NULL;
  struct MinHeapNode* root = minHeap->array[0];
  struct MinHeapNode* lastNode = minHeap->array[minHeap->size - 1];
  minHeap->array[0] = lastNode;
  minHeap->pos[root->v] = minHeap->size - 1;
  minHeap->pos[lastNode->v] = 0;
  --minHeap->size;
  minHeapify(minHeap, 0);
  return root;
}
void decreaseKey(struct MinHeap* minHeap, int v, int key) {
  int i = minHeap->pos[v];
  minHeap->array[i]->key = key;
  while (i && minHeap->array[i]->key < minHeap->array[(i - 1) / 2]->key) {
    minHeap->pos[minHeap->array[i]->v] = (i - 1) / 2;
    minHeap->pos[minHeap->array[(i - 1) / 2]->v] = i;
    swapMinHeapNode(&minHeap->array[i], &minHeap->array[(i - 1) / 2]);
    i = (i - 1) / 2;
  }
}
bool isInMinHeap(struct MinHeap* minHeap, int v) {
  if (minHeap->pos[v] < minHeap->size) return true;
  return false;
}
void printArr(int arr[], int n) {
  for (int i = 1; i < n; ++i) printf("%d - %d\n", arr[i], i);
}
vector<int> PrimMST(struct Graph* graph) {
  int V = graph->V;
  int parent[V];
  int key[V];
  struct MinHeap* minHeap = createMinHeap(V);
  for (int v = 1; v < V; ++v) {
    parent[v] = -1;
    key[v] = INT_MAX;
    minHeap->array[v] = newMinHeapNode(v, key[v]);
    minHeap->pos[v] = v;
  }
  key[0] = 0;
  minHeap->array[0] = newMinHeapNode(0, key[0]);
  minHeap->pos[0] = 0;
  minHeap->size = V;
  while (!isEmpty(minHeap)) {
    struct MinHeapNode* minHeapNode = extractMin(minHeap);
    int u = minHeapNode->v;
    struct AdjListNode* pCrawl = graph->array[u].head;
    while (pCrawl != NULL) {
      int v = pCrawl->dest;
      if (isInMinHeap(minHeap, v) && pCrawl->weight < key[v]) {
        key[v] = pCrawl->weight;
        parent[v] = u;
        decreaseKey(minHeap, v, key[v]);
      }
      pCrawl = pCrawl->next;
    }
  }
  vector<int> parent1(V, -1);
  for (int i = 1; i < V; i++) parent1[i] = parent[i];
  return parent1;
}
int main() {
  int n;
  cin >> n;
  string s1, s2;
  cin >> s1;
  cin >> s2;
  int V = 26;
  vector<int> arr;
  struct Graph* graph = createGraph(V);
  for (int i = 0; i < n; i++) {
    addEdge(graph, s1[i] - 'a', s2[i] - 'a', 1);
  }
  arr = PrimMST(graph);
  int ret = 0;
  for (int i = 1; i < 26; i++) {
    if (arr[i] != -1) ret++;
  }
  cout << ret << endl;
  for (int i = 1; i < 26; i++) {
    if (arr[i] != -1)
      cout << (char)('a' + i) << " " << (char)('a' + arr[i]) << endl;
  }
  return 0;
}
