/*

 ━━━━━┒ギリギリ♂ eye！

 ┓┏┓┏┓┃キリキリ♂ mind！

 ┛┗┛┗┛┃＼○／

 ┓┏┓┏┓┃ /

 ┛┗┛┗┛┃ノ)

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┛┗┛┗┛┃

 ┓┏┓┏┓┃

 ┃┃┃┃┃┃

 ┻┻┻┻┻┻

 */

 #include <algorithm>

 #include <iostream>

 #include <iomanip>

 #include <cstring>

 #include <climits>

 #include <complex>

 #include <fstream>

 #include <cassert>

 #include <cstdio>

 #include <bitset>

 #include <vector>

 #include <deque>

 #include <queue>

 #include <stack>

 #include <ctime>

 #include <set>

 #include <map>

 #include <cmath>

 using namespace std;

 #define fr first

 #define sc second

 #define cl clear

 #define BUG puts("here!!!")

 #define W(a) while(a--)

 #define pb(a) push_back(a)

 #define Rint(a) scanf("%d", &a)

 #define Rll(a) scanf("%I64d", &a)

 #define Rs(a) scanf("%s", a)

 #define Cin(a) cin >> a

 #define FRead() freopen("in", "r", stdin)

 #define FWrite() freopen("out", "w", stdout)

 #define Rep(i, len) for(int i = 0; i < (len); i++)

 #define For(i, a, len) for(int i = (a); i < (len); i++)

 #define Cls(a) memset((a), 0, sizeof(a))

 #define Clr(a, x) memset((a), (x), sizeof(a))

 #define Full(a) memset((a), 0x7f7f, sizeof(a))

 #define pi 3.14159265359

 #define RT return

 #define lowbit(x) x & (-x)

 #define onenum(x) __builtin_popcount(x)

 typedef long long LL;

 typedef long double LD;

 typedef unsigned long long ULL;

 typedef pair<int, int> pii;

 typedef pair<string, int> psi;

 typedef map<string, int> msi;

 typedef vector<int> vi;

 typedef vector<LL> vl;

 typedef vector<vl> vvl;

 typedef vector<bool> vb;

 const int maxn = ;

 int n, k;

 int a[maxn];

 char s[maxn];

 int main() {

     // FRead();

     while(~Rint(n) && ~Rint(k)) {

         Rs(s+);

         For(i, , n+) {

             if(s[i] == 'a') a[i] = ;

             else a[i] = ;

         }

         int l = , r = ;

         int cur = , ans = ;

         while() {

             if(r > n) break;

             while(r <= n && cur <= k) {

                 if(a[r] == ) {

                     if(cur == k) break;

                     cur++;

                 }

                 r++;

             }

             if(r - l > ans) ans = r - l;

             while(l <= r && a[l] == ) l++;

             cur--;

             l++;

         }

         int ret = ans;

         For(i, , n+) {

             if(s[i] == 'a') s[i] = 'b';

             else s[i] = 'a';

         }

         For(i, , n+) {

             if(s[i] == 'a') a[i] = ;

             else a[i] = ;

         }

         l = , r = ; cur = , ans = ;

         while() {

             if(r > n) break;

             while(r <= n && cur <= k) {

                 if(a[r] == ) {

                     if(cur == k) break;

                     cur++;

                 }

                 r++;

             }

             if(r - l > ans) ans = r - l;

             while(l <= r && a[l] == ) l++;

             cur--;

             l++;

         }

         printf("%d\n", max(ans, ret));

     }

     RT ;

 }