#line 2 "/Users/noya2/Desktop/Noya2_library/template/template.hpp"
using namespace std;

#include<bits/stdc++.h>
#line 1 "/Users/noya2/Desktop/Noya2_library/template/inout_old.hpp"
namespace noya2 {

template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p){
    os << p.first << " " << p.second;
    return os;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p){
    is >> p.first >> p.second;
    return is;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v){
    int s = (int)v.size();
    for (int i = 0; i < s; i++) os << (i ? " " : "") << v[i];
    return os;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v){
    for (auto &x : v) is >> x;
    return is;
}

void in() {}
template <typename T, class... U>
void in(T &t, U &...u){
    cin >> t;
    in(u...);
}

void out() { cout << "\n"; }
template <typename T, class... U, char sep = ' '>
void out(const T &t, const U &...u){
    cout << t;
    if (sizeof...(u)) cout << sep;
    out(u...);
}

template<typename T>
void out(const vector<vector<T>> &vv){
    int s = (int)vv.size();
    for (int i = 0; i < s; i++) out(vv[i]);
}

struct IoSetup {
    IoSetup(){
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(15);
        cerr << fixed << setprecision(7);
    }
} iosetup_noya2;

} // namespace noya2
#line 1 "/Users/noya2/Desktop/Noya2_library/template/const.hpp"
namespace noya2{

const int iinf = 1'000'000'007;
const long long linf = 2'000'000'000'000'000'000LL;
const long long mod998 =  998244353;
const long long mod107 = 1000000007;
const long double pi = 3.14159265358979323;
const vector<int> dx = {0,1,0,-1,1,1,-1,-1};
const vector<int> dy = {1,0,-1,0,1,-1,-1,1};
const string ALP = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string alp = "abcdefghijklmnopqrstuvwxyz";
const string NUM = "0123456789";

void yes(){ cout << "Yes\n"; }
void no(){ cout << "No\n"; }
void YES(){ cout << "YES\n"; }
void NO(){ cout << "NO\n"; }
void yn(bool t){ t ? yes() : no(); }
void YN(bool t){ t ? YES() : NO(); }

} // namespace noya2
#line 2 "/Users/noya2/Desktop/Noya2_library/template/utils.hpp"

#line 6 "/Users/noya2/Desktop/Noya2_library/template/utils.hpp"

namespace noya2{

unsigned long long inner_binary_gcd(unsigned long long a, unsigned long long b){
    if (a == 0 || b == 0) return a + b;
    int n = __builtin_ctzll(a); a >>= n;
    int m = __builtin_ctzll(b); b >>= m;
    while (a != b) {
        int mm = __builtin_ctzll(a - b);
        bool f = a > b;
        unsigned long long c = f ? a : b;
        b = f ? b : a;
        a = (c - b) >> mm;
    }
    return a << std::min(n, m);
}

template<typename T> T gcd_fast(T a, T b){ return static_cast<T>(inner_binary_gcd(std::abs(a),std::abs(b))); }

long long sqrt_fast(long long n) {
    if (n <= 0) return 0;
    long long x = sqrt(n);
    while ((x + 1) * (x + 1) <= n) x++;
    while (x * x > n) x--;
    return x;
}

template<typename T> T floor_div(const T n, const T d) {
    assert(d != 0);
    return n / d - static_cast<T>((n ^ d) < 0 && n % d != 0);
}

template<typename T> T ceil_div(const T n, const T d) {
    assert(d != 0);
    return n / d + static_cast<T>((n ^ d) >= 0 && n % d != 0);
}

template<typename T> void uniq(std::vector<T> &v){
    std::sort(v.begin(),v.end());
    v.erase(unique(v.begin(),v.end()),v.end());
}

template <typename T, typename U> inline bool chmin(T &x, U y) { return (y < x) ? (x = y, true) : false; }

template <typename T, typename U> inline bool chmax(T &x, U y) { return (x < y) ? (x = y, true) : false; }

template<typename T> inline bool range(T l, T x, T r){ return l <= x && x < r; }

} // namespace noya2
#line 8 "/Users/noya2/Desktop/Noya2_library/template/template.hpp"

#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define repp(i,m,n) for (int i = (m); i < (int)(n); i++)
#define reb(i,n) for (int i = (int)(n-1); i >= 0; i--)
#define all(v) (v).begin(),(v).end()

using ll = long long;
using ld = long double;
using uint = unsigned int;
using ull = unsigned long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using pil = pair<int,ll>;
using pli = pair<ll,int>;

namespace noya2{

/*　~ (. _________ . /)　*/

}

using namespace noya2;


#line 2 "c.cpp"

#line 2 "/Users/noya2/Desktop/Noya2_library/misc/clamper.hpp"

#line 5 "/Users/noya2/Desktop/Noya2_library/misc/clamper.hpp"

namespace noya2 {

template<typename T>
struct clamper {
    static constexpr T lim_l = std::numeric_limits<T>::min() / 3;
    static constexpr T lim_r = std::numeric_limits<T>::max() / 3;
    // l <= r must be hold (not checked)
    T a, l, r;
    // identity
    constexpr clamper () : a(0), l(lim_l), r(lim_r) {}
    // add only
    constexpr clamper (T _a) : a(_a), l(lim_l), r(lim_r) {}
    // clamp only
    constexpr clamper (T _l, T _r) : a(0), l(_l), r(_r) {}
    // add and clamp
    constexpr clamper (T _a, T _l, T _r) : a(_a), l(_l), r(_r) {}
    static constexpr clamper add_chmax(T _a, T _l){
        return clamper(_a, _l, lim_r);
    }
    static constexpr clamper add_chmin(T _a, T _r){
        return clamper(_a, lim_l, _r);
    }
    static constexpr clamper chmax(T _l){
        return clamper(0, _l, lim_r);
    }
    static constexpr clamper chmin(T _r){
        return clamper(0, lim_l, _r);
    }
    constexpr T eval(T x) const {
        return std::clamp(x + a, l, r);
    }
    // (f o g)(x) = f(g(x))
    friend constexpr clamper composition(const clamper &f, const clamper &g){
        return clamper(f.a + g.a, f.eval(g.l), f.eval(g.r));
    }
};

} // namespace noya2
#line 4 "c.cpp"

void solve(){
    int n; in(n);
    ll tar; in(tar);
    string s; in(s);
    auto val = [&](char c){
        if (c == '0'){
            return clamper<ll>(-1);
        }
        if (c == '1'){
            return clamper<ll>(1, -linf, 1200);
        }
        abort();
    };
    vector<clamper<ll>> rui(n+1);
    rep(i,n){
        rui[i+1] = composition(val(s[i]),rui[i]);
    }
    vector<ll> cnt(n+1);
    rep(i,n){
        cnt[i+1] = cnt[i] + (s[i] == '0' ? 1 : 0);
    }
    auto pow_clamper = [&](clamper<ll> clp, ll k){
        clamper<ll> ret;
        while (k){
            if (k & 1){
                ret = composition(ret, clp);
            }
            clp = composition(clp, clp);
            k >>= 1;
        }
        return ret;
    };
    auto ac = [&](ll len){
        clamper<ll> clp = composition(rui[len%n],pow_clamper(rui[n],len/n));
        ll cur = clp.eval(1200);
        // sub : a, ac : b, nop : c
        // a + b + c == len
        // -a + b == cur - 1200
        ll a = cnt[n] * (len/n) + cnt[len%n];
        ll b = cur - 1200 + a;
        return b;
    };
    ll le = 0, ri = 3e18;
    while (ri - le > 1){
        ll md = (le + ri) / 2;
        if (ac(md) >= tar){
            ri = md;
        }
        else {
            le = md;
        }
    }
    out(ri);
}

int main(){
    int t = 1; //in(t);
    while (t--) { solve(); }
}