#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
using ll = long long;
ll INF = 2e18;
template<typename T> using vc = vector<T>;
template<typename T> using vv = vc<vc<T>>;
using vi = vc<int>; using vvi = vv<int>;
using vl = vc<ll>; using vvl = vv<ll>;
using vs = vc<string>; using vvs = vv<string>;
using vb = vc<bool>; using vvb = vv<bool>;
using vmint = vc<mint>; using vvmint = vv<mint>;
#define rep(i,n) for(ll i=0; i<(n); i++)
#define drep(i,n) for(ll i=(n)-1; i>=0; i--)
#define rrep(i,n) for(ll i=1; i<=(n); i++)
#define nfor(i,a,b) for(ll i=a;i<b;i++)
#define dfor(i,a,b) for(ll i=(a)-1; i>=(b); i--)
template<class T>istream& operator>>(istream& i, vc<T>& v) {rep(j,(ll) size(v))i >> v[j]; return i; }
#define nall(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
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(a > b){ a = b; return 1; } return 0; }
#define YES cout<<"Yes"<<endl
#define NO cout<<"No"<<endl
#define YN {YES;}else{NO;}
#define ERROR cout<<-1<<endl
void print(long double x){ printf("%.20Lf\n",x);}
#define vc_cout(v){ll n = size(v);rep(i,n)cout<<v[i]<<endl;}
#define vv_cout(v){ll n = size(v);rep(i,n){rep(j,size(v[i])){cout<<' '<<v[i][j];}cout<<endl;}}


int main(){
    ll N, A;
    cin >> N >> A;
    string S;
    cin >> S;
    vl pref1(N+1,0);
    vl prefS(N+1,0);
    vl minpref(N+1,0);
    rep(i,N) {
        int s;
        if(S[i]=='0') s = 1;
        else s = -1;
        prefS[i+1] = prefS[i] + s;
        if(S[i]=='1') {
            pref1[i+1] = pref1[i] + 1;
        } else {
            pref1[i+1] = pref1[i];
        }
        minpref[i+1] = min(minpref[i],prefS[i+1]);
    }
    ll o = pref1[N];
    ll delta = prefS[N];
    ll mstar = minpref[N];
    auto AC = [&](ll t) -> ll {
        if(t==0) return 0;
        ll K = t/N;
        int r  = (int)(t%N);
        ll ones = (ll) K * o + pref1[r];
        ll part2 = (ll) ((ll)K * delta + minpref[r]);
        ll min_complete;
        if(K==0){
            min_complete = (ll)LLONG_MAX/4;
        }else{
            if(delta >= 0) min_complete = mstar;
            else           min_complete = (long long)((ll)(K-1) * delta + mstar);
        }
        ll minPrefix = min(min_complete, part2);

        return ones + minPrefix;
    };
    unsigned long long lo = 1, hi = 2000000000000000000ULL; // 2e18
    while(lo < hi){
        unsigned long long mid = lo + ((hi - lo) >> 1);
        if(AC(mid) >= (ll)A) hi = mid;
        else lo = mid + 1;
    }
    cout << lo << "\n";
    return 0;
}