#include <bits/stdc++.h>
using namespace std;
#pragma warning(disable : 4996)
#pragma comment(linker, "/STACK:20000007")
#pragma GCC optimize("unroll-loops")
const int N = (int)1e6 + 100;
const int maxn = (int)1e3 + 100;
const int base = (int)1e9;
const int mod = (int)1e9 + 7;
const int inf = INT_MAX;
const long long ll_inf = LLONG_MAX;
const long double pi = acos((long double)-1.0);
const long double eps = 1e-8;
void yes() { exit(!(printf("YES\n"))); }
void no() { exit(!(printf("NO\n"))); }
void solve() {
  string s;
  cin >> s;
  int n = s.size();
  if (n < 4) no();
  set<char> S;
  vector<int> Cnt(26);
  for (char c : s) Cnt[c - 'a']++, S.insert(c);
  int L = S.size();
  if (L > 4) no();
  if (3 <= L && L <= 4) yes();
  if (L == 1) no();
  if (L == 2) {
    if (!any_of(Cnt.begin(), Cnt.end(),
                [&](int Count) -> bool { return Count >= 2; }))
      no();
    vector<int> others;
    for (int i = 0; i < 26; ++i)
      if (Cnt[i] >= 2) others.push_back(i);
    if (others.size() >= 2)
      yes();
    else
      no();
  }
  no();
}
int main() {
  int T = 1;
  for (; T; --T) solve();
  return 0;
}
