#include <iostream>
#include <set>
#include <string>
using namespace std;
int main() {
	string s;
	set<char> c{};
	cin >> s;
	for (int i = 0; i < s.size(); i++) {
		c.insert(s[i]);
	}
	cout << (s.size() == c.size() ? "yes" : "no") << endl;
}