#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
 
using namespace std;
 
#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(int i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
 
#define MOD 1000000009

int f1(int n, int k, string s){
  int ans = 0;
  long long atari = 0;
  int index = 0;
  for(;;){
    if(s[index]=='0'){
      if(atari==0){
        ans++;
      }
      else{
        atari--;
      }
    }
    if(s[index]=='1'){
      if(atari==0){
        ans++;
        atari++;
      }
    }
    if(s[index]=='2'){
      if(atari==0){
        ans++;
        atari+=2;
      }
      else{
        atari++;
      }
    }
    k--;
    if(k==0)break;
    index++;
    index%=n;
  }
  return ans;
}

int f2(int n, int k, string s){
  int ans = 0;
  int atari = 0;
  for(int i = 0; i < n; i++){
    if(s[i]=='0'){
      if(atari==0){
        ans++;
      }
      else{
        atari--;
      }
    }
    if(s[i]=='1'){
      if(atari==0){
        ans++;
        atari++;
      }
    }
    if(s[i]=='2'){
      if(atari==0){
        ans++;
        atari+=2;
      }
      else{
        atari++;
      }
    }
  }
  int d = max(0,ans-atari);
  int c = max(0,k%n-10);
  return f1(n,k-c*n,s)+c*d;
}

int main(){
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  if(n!=s.sz||k>2000000000||n>50){
    cout << -1 <<endl;
    return 0;
  }
  //cout << f1(n,k,s) << endl;
  cout << f2(n,k,s) << endl;
  return 0;
}

/*
テスト用
*/