#include <bits/stdc++.h>
using namespace std;

#define int long long

#define rep(i,s,n) for(int i = s;i<n;i++)
#define repe(i,s,n) for(int i = s;i<=n;i++)
#define rrep(i,s,n) for(int i = (n)-1;i>=(s);i--)
#define all(v) (v).begin(),(v).end()
#define pb push_back
#define fi first
#define se second
#define chmin(a,b) a=min((a),(b))
#define chmax(a,b) a=max((a),(b))
typedef long long ll;
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
typedef pair<pint,int> P1;
typedef pair<int,pint> P2;
typedef pair<pint,pint>PP;
static const ll maxLL = (ll)1 << 62;
const ll MOD=1000000007;
const ll INF=1e18;
int dx[]={0,0,1,-1};
int dy[]={-1,1,0,0};

int N,K,x,y;
string s;

void hakogai(){
    int stock=0,cost=0,i;
    rep(i,0,N){
        if(stock>0)stock--;
        else cost++;
        stock+=s[i]-'0';
    }
    x=cost;
    y=stock;
}

int hasuu(int rest,int stock){
    int cost=0,i;
    rep(i,0,rest){
        if(stock>0)stock--;
        else cost++;
        stock+=s[i]-'0';
    }
    return cost;
}

signed main(){
    cin.tie(0);
	ios::sync_with_stdio(false);
    cin>>N>>K>>s;
    if(K<N)cout<<hasuu(K,0);
    else {
        hakogai();
        if(x<=y)cout<<x;
        else cout<<x+(x-y)*(K/N-1)+hasuu(K%N,y);
    }
    cout<<"\n";
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    return 0;
}