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


const int M = 1000000007;
const int N = 3e5+9;
const int INF = 2e9+1;
const int MAXN = 100000;
const int LINF = 2000000000000000001;

//_ ***************************** START Below *******************************




string a;

int consistency(int n, int k){
	int ans = 0;

	int act = 0;
	int bct = 0;
    int pairs = 0;

	int s = 0, e = 0;

	while(e<n){
		if(a[e] == 'a') act++;
		else if(a[e] == 'b') bct++;

        if(a[e] == 'b') pairs += act;

        if(pairs <= k){
            ans = max(ans, e-s+1);
            e++;
        }
        else{
            while(s<=e && pairs > k){
                if(a[s] == 'a') act--;
                else if(a[s] == 'b') bct--;

                if(a[s] == 'a') pairs -= bct;
                s++;
            }
            ans = max(ans, e-s+1);
            e++;
        }
	}

	return ans;

}









void solve() {

	int n, k;
	cin >> n >> k;
    cin >> a;
	cout << consistency(n, k) << endl;
	
	
	
	
}





int32_t main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    int t = 1;
    while (t--) {
        solve();
    }

    return 0;
}