#include <bits/stdc++.h>
using namespace std;
int i, j, m, n, len[100005], head[100005], cnt[30], tot, t, temp;
string s[100005];
bool used[30], happen[30], found, finish[100005];
int ans[30], nxt;
int main() {
  tot = 0;
  memset(cnt, 0, sizeof(cnt));
  memset(happen, false, sizeof(happen));
  memset(head, 0, sizeof(head));
  memset(finish, false, sizeof(finish));
  scanf("%d", &n);
  for (i = 1; i <= n; i++) {
    cin >> s[i];
    len[i] = s[i].length();
    memset(used, false, sizeof(used));
    for (j = 0; j <= len[i] - 1; j++) {
      if (used[s[i][j] - 'a']) {
        printf("NO\n");
        return 0;
      }
      used[s[i][j] - 'a'] = true;
      happen[s[i][j] - 'a'] = true;
      if (j > 0) cnt[s[i][j] - 'a']++;
    }
  }
  for (i = 0; i <= 25; i++) {
    if (happen[i]) tot++;
  }
  nxt = -1;
  for (i = 1; i <= tot; i++) {
    found = false;
    if (nxt != -1) {
      if (cnt[nxt] != 0) {
        printf("NO\n");
        return 0;
      }
      j = nxt;
      nxt = -1;
      for (t = 1; t <= n; t++) {
        if (!finish[t]) {
          if (s[t][head[t]] - 'a' == j) {
            head[t]++;
            if (head[t] < len[t]) {
              cnt[s[t][head[t]] - 'a']--;
              if (nxt == -1) {
                nxt = s[t][head[t]] - 'a';
              } else {
                if (nxt != s[t][head[t]] - 'a') {
                  printf("NO\n");
                  return 0;
                }
              }
            } else {
              finish[t] = true;
            }
          }
        }
      }
      happen[j] = false;
      ans[i] = j + 'a';
    } else {
      for (j = 0; j <= 25; j++) {
        if (happen[j]) {
          if (cnt[j] == 0) {
            nxt = -1;
            for (t = 1; t <= n; t++) {
              if (!finish[t]) {
                if (s[t][head[t]] - 'a' == j) {
                  head[t]++;
                  if (head[t] < len[t]) {
                    cnt[s[t][head[t]] - 'a']--;
                    if (nxt == -1) {
                      nxt = s[t][head[t]] - 'a';
                    } else {
                      if (nxt != s[t][head[t]] - 'a') {
                        printf("NO\n");
                        return 0;
                      }
                    }
                  } else
                    finish[t] = true;
                }
              }
            }
            ans[i] = j + 'a';
            happen[j] = false;
            found = true;
          }
        }
        if (found) break;
      }
      if (!found) {
        printf("NO\n");
        return 0;
      }
    }
  }
  for (i = 1; i <= tot; i++) {
    printf("%c", ans[i]);
  }
  printf("\n");
  return 0;
}
