#include <iostream>
#include <map>
#include <cmath>
#include <set>
#include <vector>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;

    string s;
    cin >> s;

    multiset<int> ms;
    for(int i = 0; i < 26; i++) {
        ms.insert(0);
    }
    int i = 0, j = 0;
    int question_marks = 0;
    int sum = 0;
    vector<int> cnt(26, 0);
    while(j < n) {
        if(*ms.begin() <= k and *ms.rbegin() <= k) {
            if(s[j] != '?') {
                sum++;
                ms.erase(ms.find(cnt[s[j] - 'a']));
                cnt[s[j] - 'a']++;
                ms.insert(cnt[s[j] - 'a']);
            }
            else {
                question_marks++;
            }

            j++;
            if(sum + question_marks == 26 * k) {
                cout << i + 1 << endl;
                return 0;
            }
        }
        else {
            if(s[i] != '?') {
                sum--;
                ms.erase(ms.find(cnt[s[i] - 'a']));
                cnt[s[i] - 'a']--;
                ms.insert(cnt[s[i] - 'a']);
            }
            else {
                question_marks--;
            }
            i++;
            if(sum + question_marks == 26 * k) {
                cout << i << endl;
                return 0;
            }

        }
    }
    cout << -1 << endl;
    return 0;
}
// a?cdefghijklmnopqrstuvwxyza?