#include <bits/stdc++.h>
using namespace std;
inline int read(register int ans = 0, register int sgn = ' ',
                register int ch = getchar()) {
  for (; ch < '0' || ch > '9'; sgn = ch, ch = getchar())
    ;
  for (; ch >= '0' && ch <= '9'; (ans *= 10) += ch - '0', ch = getchar())
    ;
  return sgn - '-' ? ans : -ans;
}
char _Ob[1000000 + 5], *_O = _Ob, _Os[25], *_Ot;
template <typename T>
inline void Out(T x) {
  if (!x) {
    (*_O++ = '0');
    return;
  }
  if (x < 0) (*_O++ = '-'), x = -x;
  for (_Ot = _Os; x; *_Ot++ = x % 10 + '0', x /= 10)
    ;
  for (; _Ot != _Os; (*_O++ = *--_Ot))
    ;
  if (_O - _Ob >= 1000000 - 50) fwrite(_Ob, 1, _O - _Ob, stdout), _O = _Ob;
}
char s[100000 + 5], ans[27];
bool o[27];
int in[27], fa[27], n, m, c;
void DFS(int j) {
  ans[++c] = j + 'a' - 1, o[j] = false;
  if (fa[j] && o[fa[j]]) --in[fa[j]], DFS(fa[j]);
}
int main() {
  n = read();
  for (register int i = 1, j; i <= n; i++) {
    for (scanf("%s", s + 1), m = strlen(s + 1), j = 1; j <= m;
         o[s[j] -= 'a' - 1] = true, j++)
      ;
    for (j = 1; j < m; j++)
      if (fa[s[j]] && fa[s[j]] != s[j + 1]) {
        puts("NO");
        return 0;
      } else
        in[s[j + 1]] += !fa[s[j]], fa[s[j]] = s[j + 1];
  }
  for (register int j = 1; j <= 26; j++)
    if (in[j] > 1) {
      puts("NO");
      return 0;
    }
  for (register int j = 1; j <= 26; o[j] && !in[j] ? DFS(j), 0 : 0, j++)
    ;
  for (register int j = 1; j <= 26; j++)
    if (o[j] && in[j]) {
      puts("NO");
      return 0;
    }
  printf("%s\n", ans + 1);
  return 0;
}
