#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<ll>;
using VV = vector<VI>;
using VS = vector<string>;

// tourist set
template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << '\n'; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
// tourist set end

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; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define rep(i,b) FOR(i, 0, b)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<'\n'
#define p2(s, t) cout << (s) << " " << (t) << '\n'
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << '\n'
#define p_yes() p("YES")
#define p_no() p("NO")
#define SZ(x) ((int)(x).size())
#define SORT(A) sort(ALL(A))
#define RSORT(A) sort(ALL(A), greater<ll>())

void no(){p_no(); exit(0);}
void yes(){p_yes(); exit(0);}

const ll mod = 1e9 + 7;
const ll inf = 1e18;
const double PI = acos(-1);

ll vsum(VI& A){
  ll sum = 0;
  for(ll a : A){
    sum += a;
  }
  return sum;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll N, K;
    cin >> N >> K;

    string s; cin >> s;
    
    VI Z; // zero
    VI O; // one
    if(s[0]=='0'){
      O.push_back(0); // sentinel
    }

    ll cnt = 1;
    char now = s[0];
    bool zeros_turn;
    if(s[0]=='0'){
      zeros_turn = true;
    }else{
      zeros_turn = false;
    }
    FOR(i, 1, N){
      if(now!=s[i]){
        if(zeros_turn){
          Z.push_back(cnt);
        }else{
          O.push_back(cnt);
        }
        cnt = 1;
        zeros_turn = !zeros_turn;
      }else{
        cnt++;
      }
      now = s[i];
    }
    if(zeros_turn){
      Z.push_back(cnt);
    }else{
      O.push_back(cnt);
    }

    debug(Z);
    debug(O);
    O.push_back(0);

    if(Z.size()<=K){
      ll ans = vsum(Z) + vsum(O);
      p(ans);
      return 0;
    }
    
    ll z_sum = 0;
    ll o_sum = 0;
    rep(i, K){
      z_sum += Z[i];
    }
    rep(i, K+1){
      o_sum += O[i];
    }
    ll ans = z_sum + o_sum;
    debug(ans);

    // zをずらしていく
    FOR(i, K, Z.size()){
      z_sum += Z[i];
      z_sum -= Z[i-K];
      o_sum += O[i+1];
      o_sum -= O[i+1-K-1];
      ll cand = z_sum + o_sum;
      chmax(ans, cand);
    }
    p(ans);
    
    return 0;
}

Measured memory_bytes: 1372