#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int n,k;
  cin>>n>>k;
  string s;
  cin>>s;

  Int d=0;
  for(char c:s) d+=c-'0';
  auto check=
    [&](Int x)->Int{
      if(n<=d){
        if(n<=x) return 1;
        Int y=x;
        for(Int i=0;i<n;i++){
          if(y==0) return i>=k;
          y--;
          y+=s[i]-'0';
        }
        return 1;
      }
      Int t=(x-d)/(n-d);
      Int cnt=n*t;
      Int res=(x+d*t)-cnt;
      while(cnt<k){
        if(res==0) break;
        res--;        
        res+=s[cnt%n]-'0';
        cnt++;
      }
      return cnt>=k;
    };
  
  Int l=0,r=k;
  while(l+1<r){
    Int m=(l+r)>>1;
    if(check(m)) r=m;
    else l=m;
  }
  cout<<r<<endl;
  return 0;
}