STRSRCH - Editorial

#include<bits/stdc++.h>
#define ll long long
#define ld long double
#define pb push_back
#define x first
#define y second
#define fastread ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define PI (atan(1)*4)
#define mp make_pair
using namespace std;
int freq[26], ans;
string s;
int main()
{
    fastread;
    cin >> s;
    for (int i = 0; i < s.size(); i++)
        freq[s[i] - 'a'] ^= 1;
    for (int i = 0; i < 26; i++)
        ans += freq[i];
    cout << ans / 2;
    return 0;
}