codeforces-1066 B Heaters

#include <cstdio>
#include <cstring>
#include <vector>
#include <iostream>
#include <algorithm>
#include <set>
#include <map>
#include <stack>

using namespace std;

#define _for(i,a,b) for(int i = (a);i < (b);i ++)
#define _rep(i,a,b) for(int i = (a);i <= (b);i ++)
typedef long long int ll;

struct cmp
{
    inline bool operator() (const int a,const int b)
    {
        return a >= b;
    }
};

int readint()
{
    int x;    //vector<int> v;v.push_back(readint())
    scanf("%d",&x);
    return x;
}

struct edge
{
    int Left;
    int Right;
};

int main()
{
    int n,r;
    while(~scanf("%d %d",&n,&r))
    {
        vector<pair<int,int>> v;
        int heaterNum = 0;
        for(int i = 0;i < n;i ++)
        {
            int tmp;
            scanf("%d",&tmp);
            if(tmp)
            {
                heaterNum ++;
                int left,right;
                left = i-r+1;
                right = i+r-1;
                if(i-r+1<0)
                {
                    left = 0;
                }
                if(i+r-1>=n)
                {
                    right = n-1;
                }
                v.push_back(make_pair(left,right));
            } 
        }
        
        sort(v.begin(),v.end());
    //    for(auto p:v)
    //        cout << p.first << " " << p.second << endl;
        int test[n] {0};
        for(int i = 0;i < v.size();i ++)
        {
            for(int j = v[i].first;j <= v[i].second;j ++)
                test[j] = 1;
        }
        int flag = 0;
        for(auto d:test)
        {
            if(!d)
            {
                flag = 1;    
            }
        }
        if(flag)
        {
            cout << "-1" << endl;
            continue;
        }
        
        int m[n] {0};
        int result = 0;
        for(int i = 0;i < n;i ++)
        {
            if(m[i]==0)
            {
                for(int j = v.size()-1;j >= 0;j --)
                {
                    if(v[j].first<=i)
                    {
                        result ++;
                        for(int k = v[j].first;k <= v[j].second;k ++)
                            m[k] = 1;
                        break;
                    }
                }
            }
        }
        cout << result << endl;
    }
    return 0;
}