#include "bits/stdc++.h"
using namespace std;
#define mod 1000000007
using ll = long long;
int dd[] = { 0, 1, 0, -1, 0 }; //→↓←↑

void solve()
{
	string S;
	int A[26] = {};
	cin >> S;
	for (int i = 0; i < S.size(); i++)
	{
		A[S[i] - 'a']++;
	}
	int odd = 0, even = 0;
	for (int i = 0; i < 26; i++)
	{
		if (A[i] % 2 == 1)
		{
			odd++;
		}
	}
	if (odd <= 1)
	{
		cout << 0 << endl;
	}
	else
	{
		cout << (odd / 2) << endl;
	}
}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	solve();
	return 0;
}
