dp = [0]
n, d = map(int, input().split())
s = input()
for i in range(1, len(s)):
if s[i] == '0':
dp.append(10 ** 10)
else:
mn = 10 ** 10
for j in range(max(0, i - d), i):
mn = min(mn, dp[j])
if mn == 10 ** 10:
dp.append(10 ** 10)
else:
dp.append(mn + 1)
abc = 1
if dp[-1] == 10 ** 10:
print(-1)
else:
print(dp[-1])
#include<bits/stdc++.h>
using namespace std;
bool dfs(string &save, int start, int dis, int m){
//cout<<start<<'\n';
if(start >= save.size() - 1){
if(start == save.size() - 1){
// cout<<start<<' '<<dis<<'\n';
cout<<dis;
return true;
}
else if(start >= save.size()) return false;
}
else{
if(save[start] == '0') return false;
for(int i = m; i >= 1; i--){
if(dfs(save, start + i, dis + 1, m)) return true;
}
}
return false;
}
int main(){
int n, m;
cin>>n>>m;
string save;
cin>>save;
int c = 0;
for(int i = 0; i < save.size(); i++){
if(c == m){
cout<<-1;
return 0;
}
if(save[i] == '1') c = 0;
else c++;
}
if(!dfs(save, 0, 0, m)) cout<<-1;
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