#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define maxn 100009
using namespace std;
char s[maxn];
int ans,len,n,pre[30],sum[maxn][30];
struct xx{
    int l,r;
}e[maxn];
bool comp(xx a,xx b){
    return a.l<b.l;
}
int main(){
	freopen("xxxx.in","r",stdin);
	freopen("xxxx.out","w",stdout);
    scanf("%s",s+1);
	len=strlen(s+1);
    for(int i=1;i<=len;i++){
        int k=s[i]-'a'+1;
        for(int j=1;j<=26;j++)sum[i][j]=sum[i-1][j];
        sum[i][k]++;
        if(pre[k]){
            e[++n].l=pre[k];
            e[n].r=i;
            pre[k]=0;
            continue;
        }
        pre[k]=i;
    }
    sort(e+1,e+n+1,comp);
    for(int i=1;i<=n;i++){
        int l=e[i].l,r=e[i].r;
        for(int j=1;j<=26;j++){
            if(sum[l-1][j]&1)
        	    if(sum[r-1][j]-sum[l][j])
				ans++;
        }
    }
    cout<<ans<<endl;
    return 0;
}