#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
#define maxn 500000 + 10
using namespace std;
char s[maxn];
int vis[maxn],cnt;
int main()
{

	freopen("xxxx.in","r",stdin);
	freopen("xxxx.out","w",stdout);
	scanf("%s",s);
	int i = 1,j = 0;
	
	while(j < strlen(s))
	{
		vis[(int)s[j]] ++;
		while(vis[(int)s[j]] % 2 == 1 && j < strlen(s))
		{
			j++;
			vis[(int)s[j]] ++;
		}
		int k = i;
		while(i < j)
		{
			if(vis[(int)s[i]] % 2 == 1) 
			   vis[(int)s[i]]++,cnt++;
			   i++;
		}
		j = k;
	}
	printf("%d",cnt / 2);
    return 0;
}