#include "bits/stdc++.h"
using namespace std;

/* macro */
#define rep(i,a,b) for(int i=a;i<b;i++)
#define revrep(i,a,b) for(int i = a; i > b; i--)
#define int long long
#define exist(s,e) ((s).find(e)!=(s).end())
#define all(v) (v).begin(), (v).end()
#define each(s,itr) for(auto (itr) = s.begin(); (itr) != s.end(); (itr)++)
#define sum(v) accumulate(all(v), (0LL))
#define isin(a, b, c) (b <= a && a <= c)
#define println cout << "\n";
#define sz(v) (int)v.size()
#define bin(x) static_cast<bitset<16> >(x)


/* alias */
template<class T> using vec = vector<T>;
typedef vector<int> vi;
typedef pair<int, int> pi;
typedef tuple<int, int, int> ti;
typedef map<int, int> mi;
typedef set<int> si;


/* constant */
const int inf = 1LL << 62;
const int mod = 1e9 + 7;
const int dx[8]={1,0,-1,0,-1,1,-1,1};
const int dy[8]={0,1,0,-1,-1,-1,1,1};
const string alphabet = "abcdefghijklmnopqrstuvwxyz";

/* io_method */
int input(){int tmp;cin >> tmp;return tmp;}
string raw_input(){string tmp;cin >> tmp;return tmp;}
template<class T> void printx(T n){cout << n;}
template<class T, class U> void printx(pair<T, U> p){cout << "(" << p.first << "," << p.second << ")";}
template<class T, class U, class V> void printx(tuple<T, U, V> t){cout << "{" << get<0>(t) << "," << get<1>(t) <<"," << get<2>(t) << "}" << endl;}
template<class T> void printx(vector<T> v){cout << "{";rep(i,0,v.size()){printx(v[i]);if(i != v.size()-1)printx(",");}cout << "}";}
template<class T> void print(T n){printx(n);cout << endl;}
template<class T> void print(set<T> s){cout << "{";each(s, e){if(e != s.begin()) printx(",");printx(*e);}cout << "}" << endl;}
template<class T, class U> void print(map<T, U> mp){cout << "{";each(mp, e){cout << "(" << e -> first << "," << e -> second << ")";}cout << "}" << endl;}
template<class T> void printans(vec<T> v){rep(i,0,sz(v)){cout << v[i] << (i == sz(v) - 1 ? "" : " ");}cout << endl;}


/* general_method */
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<class T>T cut(T &a, int l, int r){return T(a.begin()+l, a.begin()+r);}


/* math_method */
int ceil(int a, int b){return a / b + (a % b > 0);}


/* main */

struct cumsum1d{
    int n;
    vector<int> arr;

    cumsum1d(int len){
        n = len;
        rep(i,0,n) arr.push_back(0);
    }

    void init(vector<int> vec){
        arr = vec;
        n = arr.size();
    }

    void add(int a, int b, int val = 1){
        arr[a] += val;
        if(b + 1 < n) arr[b + 1] -= val;
    }

    void build(){
        rep(i,1,n) arr[i] += arr[i-1];
    }

    int query(int from, int to){
        int ret = arr[to];
        if(from) ret -= arr[from - 1];
        return ret;
    }

};



int n,k;
string s;
void in(){
    cin >> n >> k >> s;
}
vi segs;

cumsum1d cs0(0);
cumsum1d cs1(0);

void solve(){

    rep(i,0,n){
        int r = i;
        while(r < n && s[i] == s[r]) r++;
        segs.push_back(r - i);
        i = r - 1;
    }

    vi  cnt0(sz(segs), 0);
    vi cnt1(sz(segs), 0);
    if(s[0] == '0'){
        rep(i,0,sz(segs)){
            if(i % 2 == 0){
                cnt0[i] = segs[i];
            }else{
                cnt1[i] = segs[i];
            }
        }
    }else{
        rep(i,0,sz(segs)){
            if(i % 2 == 0){
                cnt1[i] = segs[i];
            }else{
                cnt0[i] = segs[i];
            }
        }
    }

    cs0.init(cnt0);
    cs1.init(cnt1);

    cs0.build();
    cs1.build();


    int ans = 0;
    int p = (s[0] == '1' ? 1 : 0);

    rep(i,0,sz(segs)){

        int cnt = 0;
        int right;
        if(p == 1){
            cnt += cs1.query(i, min(sz(cnt0) - 1, i + (k * 2)));
            cnt += cs0.query(i, min(sz(cnt0) - 1, i + (k * 2)));
            right = i + k * 2;
        }else{
            cnt += cs1.query(i, min(sz(cnt0) - 1, i + k * 2 - 1));
            cnt += cs0.query(i, min(sz(cnt0) - 1, i + (k - 1) * 2));
            right = i + k * 2 - 1;
        }
//        cout << p << ' ' << i << ' ' << cnt <<' ' << min(sz(segs) - 1, right) << endl;
        p ^= 1;
        chmax(ans, cnt);

    } 
    print(ans);




}

signed main(){

    cin.tie(0);
    ios::sync_with_stdio(false);
    in();
    solve();
    
}

Measured memory_bytes: 5044