#include <iostream>
#include <vector>
#include <map>
#include <cmath>
#include <algorithm>
#include <queue>
#include <unordered_set>
#include <unordered_map>

using namespace std;

class Solver
{
private:
	string str;
	unordered_map<int, unordered_map<char, int> > um;
public:
	Solver() {
		cin >> str;
	}

	void exec() {
		for (int i = 0; i < str.size(); ++i)
		{
			um[i][str[i]]++;
			if(i != st.size() - 1) {
				um[i+1] = um[i];
			}
		}
		int ans = 0;
		for (auto itr : um[str.size() - 1]) {
			if(itr.second % 2 == 1) {
				ans++;
			}
		}
		cout << ans << endl;
	}
	
};

int main() {
	Solver ce = Solver();
	ce.exec();
}
