#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define valid(y,x,h,w) (0<=y&&y<h&&0<=x&&x<w)
#define tpl(...) make_tuple(__VA_ARGS__)
const int INF = 0x3f3f3f3f;
const double EPS = 1e-8;
const double PI = acos(-1);
typedef long long ll;
typedef pair<int,int> pii;
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>ostream&operator<<(ostream &o,const vector<T>&t){o<<'[';FOR(i,t){if(i!=t.begin())o<<',';o<<*i;}return o<<']';}
template<class S,class T>ostream&operator<<(ostream &o,const pair<S,T>&t){return o<<'('<<t.first<<','<<t.second<<')';}
template<int N,class Tp>void output(ostream&,const Tp&){}
template<int N,class Tp,class,class ...Ts>void output(ostream &o,const Tp&t){if(N)o<<',';o<<get<N>(t);output<N+1,Tp,Ts...>(o,t);}
template<class ...Ts>ostream&operator<<(ostream&o,const tuple<Ts...>&t){o<<'(';output<0,tuple<Ts...>,Ts...>(o,t);return o<<')';}
template<class T>void output(T t,char z=10){if(t<0)t=-t,putchar(45);int c[20];
int k=0;while(t)c[k++]=t%10,t/=10;for(k||(c[k++]=0);k;)putchar(c[--k]^48);putchar(z);}
template<class T>void outputs(T t){output(t);}
template<class S,class ...T>void outputs(S a,T...t){output(a,32);outputs(t...);}
template<class T>void output(T *a,int n){REP(i,n)output(a[i],i!=n-1?',':10);}
template<class T>void output(T *a,int n,int m){REP(i,n)output(a[i],m);}
template<class T>bool input(T &t){int n=1,c;for(t=0;!isdigit(c=getchar())&&~c&&c-45;);
if(!~c)return 0;for(c-45&&(n=0,t=c^48);isdigit(c=getchar());)t=10*t+c-48;t=n?-t:t;return 1;}
template<class S,class ...T>bool input(S&a,T&...t){input(a);return input(t...);}
template<class T>bool inputs(T *a, int n) { REP(i,n) if(!input(a[i])) return 0; return 1;}

int p[50];
int d[50];
int num[50];
int nd[50];

int n,k;
string s;
bool inf[100];

int solve() {
  REP(i,n) inf[i] = 0;
  REP(i,n) {
    int t = 1;
    int a = i;
    int cnt = 0;
    while(t) {
      t--;
      t += s[a] - '0';
      a = (a+1)%n;
      cnt++;
      if (a == i && t) {
        inf[i] = 1;
        break;
      }
    }
    num[i] = cnt;
    p[i] = a;
  }
  // output(p,n);
  // output(num,n);
  REP(i,n)d[i]=-1;
  d[0] = 0;
  int a = 0;
  int cnt = 0;
  int T = 0;
  int ini = 0;
  int nc = 0;
  int TT = 0;
  vector<int> v;
  while(1) {
    nc += num[a];
    v.push_back(num[a]);
    cnt++;
    if (inf[a]) return cnt;
    a = p[a];
//    cout << a << endl;

    if (nc >= k) return cnt;
    if (d[a] != -1) {
      ini = a;
      T = cnt - d[a];
      TT = nc - nd[a];
      break;
    }
    nd[a] = nc;
    d[a] = cnt;
  }
  // cout << ini << endl;
  // cout << TT << " " << T << endl;
  ll ans = d[a] + (k-nd[ini])/TT*T;
  k -= nd[ini];
  k -= k / TT * TT;
  for (int i=v[d[ini]]; ; ++i) {
    if (k<=0) return ans;
    k -= v[i];
    ans++;
  }
  return -1;
}

int main() {
  while(input(n,k)) {
    cin >> s;
    cout << solve() << endl;
  }
}