using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using static System.Console;
using static System.Math;
using static CS_Contest.Utils;
using System.Numerics;
using System.Linq.Expressions;

//using static CS_Contest.Library;

namespace CS_Contest {
	using Li = List<int>;
	using LLi = List<List<int>>;
	using Ll = List<long>;

	internal class Program {
		private static void Main(string[] args) {
			var sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
			SetOut(sw);
			new Calc().Solve();
			Out.Flush();
		}
		

		public class Calc {

			public void Solve() {
				Dictionary<char, int> dic = new Dictionary<char, int>();
				foreach (var item in ReadLine())
				{
					if(dic.ContainsKey(item))
					{
						"no".WL();
						return;
					}
					else
					{
						dic.Add(item, 1);
					}
				}
					"yes".WL();
				return;
			}
		}
	}

	public static class Utils {
		public static int ModValue = (int)(1000000007);
		public static long INF = long.MaxValue;

		public static long Mod(long x) => x % ModValue;

		public static long ModPow(long x, long n) {
			long tmp = 1; while (n != 0) { if (n % 2 == 1) { tmp = Mod(tmp * x); } x = Mod(x * x); n /= 2; }
			return tmp;
		}

		public static long DivMod(long x, long y) => Mod(x * ModPow(y, (long)(1e9 + 5)));

		public static void WL(this object obj) => WriteLine(obj);

		public static void WL(this string obj) => WriteLine(obj);

		public static void WL<T>(this IEnumerable<T> list) => list.ToList().ForEach(x => x.WL());

		public static int ReadInt() => int.Parse(ReadLine());

		public static List<int> ReadInts(char s = ' ') => ReadLine().Split(s).Where(x => x != "").Select(int.Parse).ToList();

		public static long ReadLong() => long.Parse(ReadLine());

		public static List<long> ReadLongs(char s = ' ') => ReadLine().Split(s).Select(long.Parse).ToList();

		public static void ReadMulti(out int x, out int y) {
			var i = ReadInts(' ');
			x = i[0]; y = i[1];
		}

		public static void ReadMulti(out long x, out long y) {
			var i = ReadLongs(' ');
			x = i[0]; y = i[1];
		}

		public static void ReadMulti(out int x, out int y, out int z) {
			var i = ReadInts(' ');
			x = i[0]; y = i[1]; z = i[2];
		}

		public static void ReadMulti(out int x, out int y, out int z, out int v) {
			var i = ReadInts(' ');
			x = i[0]; y = i[1]; z = i[2]; v = i[3];
		}

		public static string StringJoin<T>(this IEnumerable<T> l, string separator = "") => string.Join(separator, l);

		public static long GCD(long m, long n) {
			long tmp;
			if (m < n) { tmp = n; n = m; m = tmp; }
			while (m % n != 0) {
				tmp = n;
				n = m % n;
				m = tmp;
			}
			return n;
		}

		public static long LCM(long m, long n) => m * (n / GCD(m, n));

		public static void REP(int n, Action<int> act) {
			for (var i = 0; i < n; i++) {
				act(i);
			}
		}

		public static void RREP(int n, Action<int> act) {
			for (var i = n - 1; i >= 0; i--) {
				act(i);
			}
		}



		public static int ManhattanDistance(int x1, int y1, int x2, int y2) => Abs(x2 - x1) + Abs(y2 - y1);

		public struct IndexT<T> {
			public T Value { get; set; }
			public int Index { get; set; }

			public IndexT(T v, int i) {
				Value = v; Index = i;
			}
			public override string ToString() {
				return Value + " " + Index;
			}
		}

		public static IEnumerable<IndexT<T>> ToIndexEnumerable<T>(this IEnumerable<T> list) => list.Select((x, i) => new IndexT<T>(x, i));

		public static Queue<T> ToQueue<T>(this IEnumerable<T> iEnumerable) {
			var rt = new Queue<T>();
			foreach (var item in iEnumerable) {
				rt.Enqueue(item);
			}
			return rt;
		}

		public static IndexT<T> IndexOf<T>(this IEnumerable<T> ie, Func<IndexT<T>, IndexT<T>, IndexT<T>> func) =>
			ie.ToIndexEnumerable().Aggregate(func);

		public static Tuple<TKey, TSource> ToTuple<TKey, TSource>(this KeyValuePair<TKey, TSource> kvp) => new Tuple<TKey, TSource>(kvp.Key, kvp.Value);

		public static IEnumerable<Tuple<TKey, TSource>> ToTupleList<TKey, TSource>(this Dictionary<TKey, TSource> d) => d.Select(_ => _.ToTuple());

		public static IEnumerable<T> When<T>(this IEnumerable<T> iEnumerable, Func<T, bool> condition, Func<T,T> filterFunc) {
			var rt = iEnumerable.ToArray();
			var index = 0;
			foreach (var item in iEnumerable) {
				rt[index] = condition(item) ? filterFunc(item) : item;
				index++;
			}
			return rt;
		}

		public static T[] Range<T>(int range,Func<int,T> func) {
			var rt = new T[range];
			for (var i = 0; i < range; i++) rt[i] = func(i);
			return rt;
		}
		public static void Swap<T>(ref T x,ref T y) {
			var tmp = x;
			x = y;
			y = tmp;
		}

		public static int Count<T>(this IEnumerable<T> l, T target) => l.Count(x => x.Equals(target));
	}
}

Measured memory_bytes: 13140