#include <bits/stdc++.h>
using namespace std;
vector<int> v[508];
bool visi[505][505];
int main(int argc, char const *argv[]) {
  int n, m, y, z, flag = 0, count = 0;
  cin >> n >> m;
  memset(visi, false, sizeof(visi));
  for (int i = 0; i < m; ++i) {
    cin >> z >> y;
    visi[z][y] = true;
    visi[y][z] = true;
  }
  for (int i = 1; i <= n; ++i) {
    count = 0;
    for (int j = 1; j <= n; ++j) {
      if (visi[i][j]) {
        count++;
      }
    }
    v[count].push_back(i);
  }
  int x = v[n - 1].size();
  int a = 0, c = 0;
  for (int i = 0; i <= 505; ++i) {
    if (i == n - 1) {
      continue;
    }
    if (v[i].size() != 0 && a != 0 && c != 0 && (i != n - 1)) {
      flag = 1;
      break;
    }
    if (v[i].size() != 0) {
      if (a == 0) {
        a = i - x + 1;
        if (a != v[i].size()) {
          if (v[i].size() - a == i - x + 1) {
            c = i - x + 1;
          } else {
            flag = 1;
            break;
          }
        }
      } else if (c == 0) {
        c = i - x + 1;
        if (c != v[i].size()) {
          flag = 1;
          break;
        }
      }
    }
  }
  if (flag == 1) {
    cout << "No" << endl;
    return 0;
  }
  vector<int> v1, v2;
  for (int i = 1; i <= n; ++i) {
    for (int j = i + 1; j <= n; ++j) {
      if (visi[i][j] == false) {
        if (v1.size() == 0) {
          v1.push_back(i);
          v2.push_back(j);
        } else {
          if (visi[v1[0]][i] || v1[0] == i) {
            v1.push_back(i);
            v2.push_back(j);
          } else {
            v2.push_back(i);
            v1.push_back(j);
          }
        }
      }
    }
  }
  int arr[n + 1];
  memset(arr, 0, sizeof(arr));
  for (int i = 0; i < v1.size(); ++i) {
    arr[v1[i]] = 1;
  }
  for (int i = 0; i < v2.size(); ++i) {
    arr[v2[i]] = 2;
  }
  cout << "Yes" << endl;
  for (int i = 1; i <= n; ++i) {
    if (arr[i] == 1) {
      cout << 'a';
    } else if (arr[i] == 0) {
      cout << 'b';
    } else {
      cout << 'c';
    }
  }
  cout << endl;
  return 0;
}
