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//Made By Phuong Nam PROPTIT <3//
#pragma GCC Optimize("O3")
#include<bits/stdc++.h>
#define alphaa "abcdefghijklmnopqrstuvwxyz"
#define ALPHAA "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
#define f(i,a,b) for(int i=a;i<=b;i++)
#define f1(i,n) for(int i=1;i<=n;i++)
#define f0(i,n) for(int i=0;i<n;i++)
#define ff(i,b,a) for(int i=b;i>=a;i--)
#define el cout<<'\n'
#define fi first
#define se second
#define pb push_back
#define pk pop_back
#define vi vector<int>
#define vl vector<ll>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define all(s) s.begin(),s.end()
using namespace std;
typedef long long ll;
const int N=1e6+3;
const int MOD=1e9+7;
string s;
int n,dema=0,demb=0,ans=0,start=1;
ll val,c;
void xuly()
{
    cin>>n>>c>>s;
    s=' '+s;
    f1(i,n)
    {
        if(s[i]=='a') dema++;
        if(s[i]=='b') demb++,val+=dema;
        while(val>c)
        {
            if(s[start]=='a') dema--, val-=demb;
            if(s[start]=='b') demb--;
            start++;
        }
        ans=max(ans,i-start+1);
    }
    cout<<ans;
}
int main()
{
  ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
  int t=1;
  //cin>>t;
  while(t--) xuly();
}