#include <bits/stdc++.h>
using namespace std;
char str[100010];
int n = 0;
int nexT[27] = {0};
int last[27] = {0};
int number = 0;
int start[27] = {0};
int len = 0;
char s[100010];
bool isUsed[27] = {false};
int main(int argc, const char* argv[]) {
  cin >> n;
  bool abort = false;
  for (int i = 0; i < n; ++i) {
    scanf("%s", str);
    if (abort) {
      continue;
    }
    for (int i = 0; i <= strlen(str); ++i) {
      if (i == 0) {
        if (last[str[i] - 'a' + 1] == 0) {
          last[str[i] - 'a' + 1] = -1;
        }
        continue;
      }
      isUsed[str[i - 1] - 'a' + 1] = true;
      if (i == strlen(str)) {
        continue;
      }
      if (nexT[str[i - 1] - 'a' + 1] == 0 && last[str[i] - 'a' + 1] != 1) {
        last[str[i] - 'a' + 1] = 1;
        nexT[str[i - 1] - 'a' + 1] = str[i] - 'a' + 1;
      } else if (nexT[str[i - 1] - 'a' + 1] == str[i] - 'a' + 1) {
        continue;
      } else {
        abort = true;
        break;
      }
    }
  }
  for (int i = 1; i <= 26; ++i) {
    if (last[i] == -1) {
      start[number++] = i;
    }
  }
  if (abort || number == 0) {
    cout << "NO" << endl;
  } else {
    for (int i = 0; i < number; ++i) {
      int pos = start[i];
      s[len++] = pos + 'a' - 1;
      isUsed[pos] = false;
      while (nexT[pos] != 0) {
        pos = nexT[pos];
        isUsed[pos] = false;
        s[len++] = pos + 'a' - 1;
      }
    }
    s[len++] = '\0';
    for (int i = 1; i <= 26; ++i) {
      if (isUsed[i]) {
        abort = true;
      }
    }
    if (abort) {
      cout << "NO" << endl;
    } else {
      cout << s << endl;
    }
  }
}
