#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long INFF = 0x3f3f3f3f3f3f3f3fll;
const int MAX = 3e5 + 10;
const long long MOD = 1e9 + 7;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
template <typename T>
inline T abs(T a) {
  return a > 0 ? a : -a;
}
template <class T>
inline void read(T& num) {
  bool start = false, neg = false;
  char c;
  num = 0;
  while ((c = getchar()) != EOF) {
    if (c == '-')
      start = neg = true;
    else if (c >= '0' && c <= '9') {
      start = true;
      num = num * 10 + c - '0';
    } else if (start)
      break;
  }
  if (neg) num = -num;
}
inline long long powMM(long long a, long long b, long long M) {
  long long ret = 1;
  a %= M;
  while (b) {
    if (b & 1) ret = ret * a % M;
    b >>= 1;
    a = a * a % M;
  }
  return ret;
}
void open() { freopen("out.txt", "w", stdout); }
char a[MAX];
int id(char x) { return x - 'a'; }
int now;
int an;
int main() {
  scanf("%s", a);
  int len = strlen(a);
  for (int i = 0; i < len; i++) {
    now ^= (1 << id(a[i]));
  }
  while (now) {
    if (now & 1) ++an;
    now >>= 1;
  }
  printf("%d\n", max(1, an));
  return 0;
}
