#include<bits/stdc++.h>

using namespace std;

int main()

{

string a;

while(cin >> a)

{

int c=1, t=0;

for(int i=0; i<a.length(); i++)

{

if(('a'<=a[i] and a[i]<='z') or ('A'<=a[i] and a[i]<='Z'))

{

for(int j=i+1; j<a.length(); j++)

{

if(tolower(a[j])==tolower(a[i]))

{

a[j]='*';

c++;

}

}

if(c%2!=0)

t++;

c=1;

}

}

if(t==1 or t==0)

cout << "yes !\n";

else

cout << "no...\n";

}

}