#include <bits/stdc++.h>
#pragma GCC optimize("O2")
const int inf = 0x3fffffff;
const int maxn = 2017;
using namespace std;
inline int read() {
  int f = 1, x = 0;
  char ch = getchar();
  while (ch > '9' | ch < '0') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch <= '9' && ch >= '0') {
    x = (x << 3) + (x << 1) + ch - '0';
    ch = getchar();
  }
  return f * x;
}
int ans;
char s[100001];
map<char, int> num;
int main() {
  scanf("%s", s + 1);
  for (int i = 1; i <= strlen(s + 1); i++) {
    num[s[i]]++;
  }
  for (int i = 0; i < 26; i++) {
    if (num[(char)('a' + i)] & 1) ans++;
  }
  if (!ans) ans = 1;
  cout << ans;
}
