ABC345-CでWAとなってしまう理由を教えてください…

#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;

int main() {
    string S;
    cin >> S;
    int charCount[30] = {0};

    for (char c : S) {
        charCount[c - 'a']++;
    }
    
    ll mns = 0;
    bool on = true;
    
    for (int i = 0; i < 26; ++i) {
        if (charCount[i] > 1) {
            mns+=charCount[i]*(charCount[i]-1)/2 ;
            on = false;
        }
    }
    
    if(on == false){
      cout << S.size() * (S.size()-1) /2 - mns +1 << endl;
      return 0;
    } else {
      cout << S.size() * (S.size()-1) /2  << endl;
      return 0;
    }
    
}