#include <bits/stdc++.h>
using namespace std;
long long int INF = 3e18;
long long int mod = 4294967296;
double Pi = 3.1415926535897932384626;
vector<long long> G[500005];
vector<pair<long long, long long> > tree[500010];
priority_queue<long long> pql;
priority_queue<pair<long long, long long> > pqp;
priority_queue<long long, vector<long long>, greater<long long> > pqls;
priority_queue<pair<long long, long long>, vector<pair<long long, long long> >,
               greater<pair<long long, long long> > >
    pqps;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, -1, 1};
char dir[] = "DRUL";
long long rui(long long abc, long long bed) {
  if (bed == 0) {
    return 1;
  } else {
    long long ced = rui(abc, bed / 2);
    ced *= ced;
    ced %= mod;
    if (bed % 2 == 1) {
      ced *= abc;
      ced %= mod;
    }
    return ced;
  }
}
long long gcd(long long number1, long long number2) {
  if (number1 > number2) {
    swap(number1, number2);
  }
  if (number1 == 0 || number1 == number2) {
    return number2;
  } else {
    return gcd(number2 % number1, number1);
  }
}
void YES(bool condition) {
  if (condition) {
    cout << "YES" << endl;
    ;
  } else {
    cout << "NO" << endl;
    ;
  }
  return;
}
void Yes(bool condition) {
  if (condition) {
    cout << "Yes" << endl;
    ;
  } else {
    cout << "No" << endl;
    ;
  }
  return;
}
long long n, m, num, sum, ans, a, b, c, d, e, f, g, h, i, j, k, w;
long long x[500005], y[500005], z[500005];
char s[500005];
int main() {
  cin >> s;
  n = strlen(s);
  for (i = 0; i < n; i++) {
    x[s[i] - 'a']++;
  }
  ans = 0;
  for (i = 0; i < 26; i++) {
    if (x[i] % 2 == 1) ans++;
  }
  cout << max(ans, 1ll) << endl;
  ;
  return 0;
}
