#include <bits/stdc++.h>
using namespace std;
const int LOG = 18;
const int NN = 2e5 + 5;
const int MOD = (int)1e9 + 7;
const int INF = 1LL << 28;
bool adj[30][30];
char clr[30];
int in_deg[30];
bool exist[30];
string ans;
bool good;
void dfs(int x) {
  if (clr[x] == 'g') {
    good = false;
    return;
  }
  if (clr[x] != 'w') return;
  clr[x] = 'g';
  ans += ((char)(x + 'a'));
  for (int y = 0; y < 30; y++)
    if (adj[x][y]) {
      dfs(y);
    }
  clr[x] = 'b';
}
char s[1 << 17];
int main() {
  ios_base::sync_with_stdio(0);
  int n;
  scanf("%d\n", &n);
  memset(adj, false, sizeof(adj));
  while (n--) {
    fgets(s, 1 << 19, stdin);
    int l = strlen(s);
    if (s[l - 1] == '\n') s[l - 1] = '\0', l--;
    for (int i = 0; i < l; i++) {
      exist[s[i] - 'a'] = true;
      if (i > 0) adj[s[i - 1] - 'a'][s[i] - 'a'] = true;
    }
  }
  good = true;
  ans.clear();
  for (int i = 0; i < 30; i++) {
    in_deg[i] = 0;
    int out = 0;
    for (int j = 0; j < 30; j++) {
      in_deg[i] += adj[j][i];
      out += adj[i][j];
    }
    if (in_deg[i] >= 2 || out >= 2) {
      good = false;
    }
  }
  memset(clr, 'w', sizeof(clr));
  for (int i = 0; i < 30; i++)
    if (exist[i] == true && in_deg[i] == 0) {
      dfs(i);
    }
  for (int i = 0; i < 30; i++)
    if (exist[i] == true && clr[i] == 'w') good = false;
  if (good)
    printf("%s\n", ans.c_str());
  else
    printf("NO\n");
  return 0;
}
