#include <bits/stdc++.h>

using namespace std;

ifstream fin  ("zombie.in");
ofstream fout ("zombie.out");

int st, md, dr;
int d, n, k, x, lst;
int v[1000005], dp[1000005];
long long s[1000005];

int main (){
    fin>>d>>n>>k>>lst;
    for(int i=1; i<n; i++){
        fin>>x;
        v[i]=x-lst;
        lst=x;
    }

    for(int i=1; i<k; i++){
        dp[i]=dp[i-1]+1;
        s[i]=s[i-1] + v[i];
    }


    for(int i=k; i<n; i++){
        x=v[i];
        s[i]=s[i-1] + x;


        st=1;
        dr=i-k+1;
        while(st <= dr){
            md=(dr-st)/2+st;
            if(s[i]-s[md-1] >= d)
                st=md+1;
            else
                dr=md-1;
        }


        //cout<<st<<" ";
        dp[i]=min(dp[i-1]+1, dp[st-1]+k);
    }

    fout<<dp[n-1];
    return 0;
}