#include <bits/stdc++.h>
using namespace std;
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
int main() {
  string s;
  cin >> s;
  long long n = s.size();
  map<char, long long> m;
  long long i;
  for (i = 0; i < n; i++) m[s[i]]++;
  long long count = 0;
  long long tot = 0;
  for (char x = 'a'; x <= 'z'; x++) {
    if (m[x] > 0) tot++;
    if (m[x] > 1) count++;
  }
  if (tot > 4 || tot == 1)
    cout << "NO" << '\n';
  else if (tot == 4)
    cout << "YES" << '\n';
  else if (tot == 3) {
    if (count >= 1)
      cout << "YES" << '\n';
    else
      cout << "NO" << '\n';
  } else {
    if (count > 1)
      cout << "YES" << '\n';
    else
      cout << "NO" << '\n';
  }
}
