#ifdef __GNUC__
#pragma GCC optimize ("O3")
#pragma GCC target ("avx")
#endif

#define _USE_MATH_DEFINES

#include <iostream>
#include <iomanip>
#include <stdio.h>

#include <sstream>
#include <algorithm>
#include <cmath>

#include <string>
#include <cstring>
#include <vector>
#include <valarray>
#include <array>

#include <queue>
#include <complex>
#include <set>
#include <map>
#include <stack>
#include <list>

#include<cassert>//assert();
#include <fstream>
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)

/////////
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
#define PII pair<int,int>
/////////
using namespace::std;

// 最大公約数
template<class T>
inline T gcd(T a, T b){return b == 0 ? a : gcd(b, a % b);}
// 最小公倍数
template<class T>
inline T lcm(T a, T b){return a * b / gcd(a, b);}
////////////////////////////////
int sm(int N,int K,char* bar,int buy,int hit,int eat){
	for(;;){
		for(int i=0; i<N; ++i){
			if( hit == 0 ){++buy;}
			else{--hit;}
			hit += bar[i] - '0';
			++eat;
			if( eat >= K ){
				return buy;
			}
		}
	}
	return -1;
}
inline void solve(){
	//scanf,printf
	char bar[51];
	int N,K;
	scanf("%d%d %s",&N,&K,bar);
	//printf("%d\n%d\n%s",N,K,bar);
	int buy=0;
	int hit=0;
	for(int i=0;i<N;++i){
		if( hit == 0 ){++buy;}
		else{--hit;}
		hit += (bar[i] - '0');
	}
	/*
	buy < hit
	buy == hit
	hit < buy
	*/
	int BuyAll,atari,Eat;
	int hako = K/N - 1;
	int ret;
	if( hit <= buy ){
		BuyAll = buy + (hako-1) * (buy-hit);
		atari = hit;
		Eat = hako * N;
		ret = sm(N,K,bar,BuyAll,atari,Eat);
		printf("%d\n",ret);
	}else{//buy < hit
		if( K <= buy ){
			ret = sm(N,K,bar,0,0,0);
			printf("%d\n",ret);
		}else{
			ret = buy;
			printf("%d\n",ret);
		}
	}
}

int main(void){
    //std::cin.tie(0);
    //std::ios::sync_with_stdio(false);
	//std::cout << std::fixed;//小数を10進数表示
    //cout << setprecision(16);//小数をいっぱい表示する。16?

	solve();

	return 0;
}