1 #include <cstdio>
  2 #include <cstring>
  3 #include <vector>
  4 #include <iostream>
  5 #include <algorithm>
  6 #include <set>
  7 #include <map>
  8 #include <stack>
  9 
 10 using namespace std;
 11 
 12 #define _for(i,a,b) for(int i = (a);i < (b);i ++)
 13
 
 #define _rep(i,a,b) for(int i = (a);i <= (b);i ++)
 14 typedef long long int ll;
 15 
 16 struct cmp
 17 {
 18     inline bool operator() (const int a,const int b)
 19     {
 20         return a >= b;
 21     }
 22 };
 23 
 24 int readint()
 25 {
 26     int x;    //vector<int> v;v.push_back(readint())
 

 27     scanf("%d",&x);
 28     return x;
 29 }
 30 
 31 struct edge
 32 {
 33     int Left;
 34     int Right;
 35 };
 36 
 37 int main()
 38 {
 39     int n,r;
 40     while(~scanf("%d %d",&n,&r))
 41     {
 42         vector<pair<int,int>> v;
 43         int heaterNum = 0;
 44
 
         for(int i = 0;i < n;i ++)
 45         {
 46             int tmp;
 47             scanf("%d",&tmp);
 48             if(tmp)
 49             {
 50                 heaterNum ++;
 51                 int left,right;
 52                 left = i-r+1;
 53                 right = i+r-1;
 54                 if(i-r+1<0)
 55                 {
 56                     left = 0;
 57                 }
 58                 if(i+r-1>=n)
 59                 {
 60                     right = n-1;
 61                 }
 62                 v.push_back(make_pair(left,right));
 63             } 
 64         }
 65         
 66         sort(v.begin(),v.end());
 67     //    for(auto p:v)
 68     //        cout << p.first << " " << p.second << endl;
 69         int test[n] {0};
 70         for(int i = 0;i < v.size();i ++)
 71         {
 72             for(int j = v[i].first;j <= v[i].second;j ++)
 73                 test[j] = 1;
 74         }
 75         int flag = 0;
 76         for(auto d:test)
 77         {
 78             if(!d)
 79             {
 80                 flag = 1;    
 81             }
 82         }
 83         if(flag)
 84         {
 85             cout << "-1" << endl;
 86             continue;
 87         }
 88         
 89         int m[n] {0};
 90         int result = 0;
 91         for(int i = 0;i < n;i ++)
 92         {
 93             if(m[i]==0)
 94             {
 95                 for(int j = v.size()-1;j >= 0;j --)
 96                 {
 97                     if(v[j].first<=i)
 98                     {
 99                         result ++;
100                         for(int k = v[j].first;k <= v[j].second;k ++)
101                             m[k] = 1;
102                         break;
103                     }
104                 }
105             }
106         }
107         cout << result << endl;
108     }
109     return 0;
110 }

codeforces-1066B Heaters