#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
const int MAXN = 100000;
const int MAXLEN = 100000;
int n;
char s[MAXLEN + 1];
int off[MAXN + 1];
int len[MAXN];
int nxt[26];
bool any[26];
bool fst[26];
bool done[26];
char ret[27];
int nret;
bool solve() {
  for (int i = (0); i < (26); ++i)
    any[i] = false, nxt[i] = -1, fst[i] = true, done[i] = false;
  for (int i = (0); i < (n); ++i)
    for (int j = (off[i]); j < (off[i + 1]); ++j) {
      int x = s[j] - 'a';
      any[x] = true;
      if (j + 1 < off[i + 1]) {
        int y = s[j + 1] - 'a';
        if (nxt[x] != -1 && nxt[x] != y) return false;
        nxt[x] = y;
        fst[y] = false;
      }
    }
  nret = 0;
  for (int i = (0); i < (26); ++i)
    if (any[i] && fst[i]) {
      int x = i;
      while (x != -1 && !done[x]) {
        ret[nret++] = 'a' + x;
        done[x] = true;
        x = nxt[x];
      }
      if (x != -1) return false;
    }
  for (int i = (0); i < (26); ++i)
    if (any[i] && !done[i]) return false;
  return true;
}
void run() {
  scanf("%d", &n);
  off[0] = 0;
  for (int i = (0); i < (n); ++i) {
    scanf("%s", s + off[i]);
    len[i] = strlen(s + off[i]);
    off[i + 1] = off[i] + len[i];
  }
  if (!solve()) {
    printf("NO\n");
    return;
  }
  printf("%s\n", ret);
}
int main() {
  run();
  return 0;
}
