#include <bits/stdc++.h>
using namespace std;
int main()
{
int k;
cin>>k;
int temp = k;
string s;
cin>>s;
int n = s.length();
// checking for longest substring of a by changing k b's
if(s[0]!='a')
{
k--;
}
char ch1 = 'a';
int max_a = -1;
int cnt = 1;
for(int i=1;i<n;i++)
{
if(s[i]==ch1)
{
cnt++;
}
else if(s[i]!=ch1 and k>0)
{
cnt++;
k--;
}
else if(s[i]!=ch1 and k==0)
{
max_a = max(max_a,cnt);
cnt = 1;
k = temp;
k--;
}
}
max_a = max(cnt,max_a);
// checking for longest substring of b by changing k a's
k = temp;
if(s[0]!='b')
{
k--;
}
char ch2 = 'b';
int max_b = -1;
cnt = 1;
for(int i=1;i<n;i++)
{
if(s[i]==ch2)
{
cnt++;
}
else if(s[i]!=ch2 and k>0)
{
cnt++;
k--;
}
else if(s[i]!=ch2 and k==0)
{
max_b = max(max_b,cnt);
cnt = 1;
k = temp;
k--;
}
}
max_b = max(cnt,max_b);
cout<<max(max_a,max_b)<<"\n";
return 0;
}