#include <iostream>
#include <cstring>
using namespace std;
int nrap[10], ma, ima;
unsigned int i;
char sir[256], cifre[11] = "0123456789";
int main ()
{
  cin.get (sir, 256);
  for (i=0;i<strlen(sir);i++)
    if (strchr(cifre,sir[i]))
      nrap[(int)sir[i]-(int)'0']++;
  for (i = 0; i<=9;i++)
    if (nrap[i] > ma)
      {
	ma = nrap[i];
	ima = i;
      }
  if (ma == 0)
    cout << "NU" << '\n';
  else
    cout << ima << '\n';
  return 0;
}