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

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
#define rep3(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define rep2(i, a, b) rep3(i, a, b, 1)
#define rep1(i, n) rep2(i, 0, n)
#define rep0(n) rep1(aaaaa, n)
#define ov4(a, b, c, d, name, ...) name
#define rep(...) ov4(__VA_ARGS__, rep3, rep2, rep1, rep0)(__VA_ARGS__)
#define per(i, a, b) for (ll i = (a) - 1; i >= (b); i--)
#define fore(e, v) for (auto&& e : v)
#define all(a) begin(a), end(a)
#define sz(a) (int)(size(a))
#define lb(v, x) (lower_bound(all(v), x) - begin(v))
#define eb emplace_back

template <typename T, typename S>
bool chmin(T& a, const S& b) {
  return a > b ? a = b, 1 : 0;
}
template <typename T, typename S>
bool chmax(T& a, const S& b) {
  return a < b ? a = b, 1 : 0;
}
const int INF = 1e9 + 100;
const ll INFL = 3e18 + 100;
#define i128 __int128_t
struct _ {
  _() { cin.tie(0)->sync_with_stdio(0), cout.tie(0); }
} __;

int main(){
  ll N,A;
  cin>>N>>A;
  string S;
  cin>>S;
  ll ans=1;
  rep(i,N){
    if(S[i]=='0'){
      ans+=i;
      S=S.substr(i,N-i)+S.substr(0,i);
      break;
    }
  }
  vi up;
  vector<pii> upif;
  int t=0,tm=0;
  rep(i,N){
    if(S[i]=='0')t--,tm--;
    else{
      if(t<0){
        up.push_back(i);
        t++;
      }else{
        upif.push_back(pii{tm,i});
      }
      tm++;
    }
  }
  if(t==0){
    ll lp=(A-1)/sz(up);
    ans+=N*lp;
    ans+=up[(A-1)%(sz(up))];
  }else{
    sort(all(upif));
    int c=0,ifidx=0;
    while(ifidx<sz(upif)){
      if(sz(up)>=A){
        sort(all(up));
        ans+=up[(A-1)%(sz(up))];
        A=0;
        break;
      }
      ans+=N;
      A-=sz(up);
      c-=t;
      while(ifidx<sz(upif)&&upif[ifidx].first<c){
        up.push_back(upif[ifidx].second);
        ifidx++;
      }
    }
    if(A>0){
      sort(all(up));
      ll lp=(A-1)/sz(up);
      ans+=N*lp;
      ans+=up[(A-1)%(sz(up))];
    }
  }
  cout<<ans<<'\n';
}