#include <bits/stdc++.h>

using namespace std;

#define FILE ""
#define FILEIO 0
#define LONG 0
#define IOSOPT 1

#if LONG
	typedef long long ll;
	typedef unsigned long long ull;
#else // LONG
	typedef int ll;
	typedef unsigned int ull;
#endif
typedef short sym;
#define pb push_back
#define mp make_pair
#define p(a, b) pair < a, b >
#define rev(a) reverse(a.begin(), a.end())
#define ord(a) sort(a.begin(), a.end())
#define rm erase
#define rs resize
#define sz size
#define fi first
#define se second
#define ins insert
#define die(a) \
	{ \
		cout << a << endl; \
		exit(0); \
	}
#define stlprint(a) \
	for(auto _STL_ITER = a.begin(); _STL_ITER != a.end(); _STL_ITER++) \
	{ \
		cout << (*_STL_ITER) << ' '; \
	}

p(ll, ll) p[100500];
char tmp[100500];

main()
{
    #if IOSOPT
        ios_base::sync_with_stdio(0);
        cin.tie(0);
    #endif // IOSOPT
    #if FILEIO
        freopen(FILE".in", "r", stdin);
        freopen(FILE".out", "w", stdout);
    #endif
    ll n, k;
    string a;
    scanf("%d %d", &n, &k);
    scanf("%s", tmp);
    a = tmp;
    for(int i = 1; i <= a.sz(); i++)
    {
        p[i] = p[i - 1];
        (a[i - 1] == 'a' ? p[i].fi : p[i].se)++;
    }
    ll l = 1, r = 1, res = -1;
    while(r <= n)
    {
        if (r < l) l = r;
        if((p[r].fi - p[l - 1].fi) <= k)
            res = max(res, (r++) - l + 1);
        else
            l++;
    }
    l = r = 1;
    while(r <= n)
    {
        if (r < l) l = r;
        if((p[r].se - p[l - 1].se) <= k)
            res = max(res, (r++) - l + 1);
        else
            l++;
    }
    l = r = 1;
    printf("%d", res);
}