MONOTONE

#include <bits/stdc++.h>
#define ll long long
using namespace std;

const int N = 1e6 + 5;

ll n, c;
string s;

int main() {
    freopen("MONOTONE.inp", "r", stdin);
    freopen("MONOTONE.out", "w", stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);

    cin >> n >> c >> s;

    ll inca = 0, incb = 0, ab = 0;
    ll ans = 0;

    ll j = 0;
    for(ll i = 0; i < n; ++i) {
        while(j < n && ab <= c) {
            ans = max(ans, (j - i) * 1LL);

            if(s[j] == 'a')
                ++inca;
            else if(s[j] == 'b')
                ++incb, ab += inca;

            ++j;
        }

        if(ab <= c) ans = max(ans, (j - i) * 1LL);

        if(s[i] == 'b')
            --incb;
        else if(s[i] == 'a')
            --inca, ab -= incb;
    }

    cout << ans << '\n';

    return 0;
}