//https://www.hackerrank.com/challenges/sherlock-and-valid-string
//https://www.hackerrank.com/contests/101hack26/challenges/sherlock-and-valid-string
import java.io.*;
import java.util.Arrays;

public class Solution {
  public static void main(String[] args) throws IOException {
    
    //Get input
    final char[] S = (new BufferedReader(new InputStreamReader(System.in))).readLine().toCharArray();
    
    //Print output
    System.out.print(isValid(S) ? "YES" : "NO");
  }
  
  private static boolean isValid(final char[] S){
    
    //If string contains less than two letters
    if(S.length < 2){
      return true;
    }
    
    //Create histogram of letters
    final int[] L = new int[26];
    for(int i = 0, N = S.length; i < N; ++L[S[i++] - 'a']){}
    
    //Sort the histogram
    Arrays.sort(L);
    
    //Ignore unseen letters
    int i = 0;
    while(i < 26 && L[i] == 0){
      ++i;
    }
    
    //If all letter frequencies are equal
    if(L[i] == L[25]){
      return true;
    }
    
    //If there's one letter above the rest by one
    if(L[25] > L[24]){
      return L[i] == L[24] && L[25] - L[24] == 1;
    }
    
    //If there's one letter below the rest by one or that can be removed
    return L[i] < L[i+1] && L[i+1] == L[25] && (L[i] == 1 || L[i+1] - L[i] == 1);
  }
}

//https://www.hackerrank.com/challenges/sherlock-and-valid-string
//https://www.hackerrank.com/contests/101hack26/challenges/sherlock-and-valid-string
import java.io.*;
import java.util.Arrays;

public class Solution {
    public static void main(String[] args) throws IOException {
        
        //Get input
        final char[] S = (new BufferedReader(new InputStreamReader(System.in))).readLine().toCharArray();
        
        //Print output
        System.out.print(isValid(S) ? "YES" : "NO");
    }
    
    private static boolean isValid(final char[] S){
        
        //If string contains less than two letters
        if(S.length < 2){
            return true;
        }
        
        //Create histogram of letters
        final int[] L = new int[26];
        for(int i = 0, N = S.length; i < N; ++L[S[i++] - 'a']){}
        
        //Sort the histogram
        Arrays.sort(L);
        
        //Ignore unseen letters
        int i;
        for(i = 0; i < 26 && L[i] == 0; ++i){}
        
        //If all letter frequencies are equal
        if(L[i] == L[25]){
            return true;
        }
        
        //If there's one letter above the rest by one
        if(L[25] > L[24]){
            return L[i] == L[24] && L[25] - L[24] == 1;
        }
        
        //If there's one letter below the rest by one or that can be removed
        return L[i] < L[i+1] && L[i+1] == L[25] && L[i] == 1;
    }
}

Update sherlock-and-valid-string.java