#include <bits/stdc++.h>
using namespace std;
using LL = long long;
int di[] = {0, -1, 0, 1};
int dj[] = {1, 0, -1, 0};
string s;
int n;
int sum[200001][(26)];
int memo[200000][2];
int solve(int l, int i) {
  int count = 0;
  for (int j = 0; j < (26); j++) {
    if ((sum[i + 1][j] - sum[l][j]) % 2 == 1) {
      count++;
    }
  }
  if (i == n - 1) {
    if (count < 2) {
      return 0;
    } else {
      return 2 * n;
    }
  }
  if (count > 1) {
    memo[i][1] = 2 * n;
  } else {
    if (memo[i][1] == -1) {
      memo[i][1] = solve(i + 1, i + 1) + 1;
    }
  }
  return min(memo[i][0], memo[i][1]);
}
int main() {
  std::ios::sync_with_stdio(false);
  cin >> s;
  n = s.size();
  memset(sum, 0, sizeof(sum));
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < (26); j++) {
      sum[i + 1][j] = sum[i][j];
    }
    sum[i + 1][s[i] - 'a']++;
  }
  int count = 0;
  for (int i = 0; i < (26); i++) {
    if (sum[n][i] % 2 == 1) {
      count++;
    }
  }
  if (count == 0) {
    cout << 1 << endl;
  } else {
    cout << count << endl;
  }
  return 0;
}
