#include <fstream>
#include <iostream>

using namespace std;

ifstream f("zombie.in");
ofstream g("zombie.out");

int d, n, k, bug;
long long rez;

void rezolva(){

    f >> d >> n >> k;

    int st = 0, dr = 0;

    f >> st;
    int i = 2;

    while(i<=n){
        f >> dr;
        if (dr - st + bug >= d){
            if (bug == 0){
                rez += 1LL;
                bug = 0;
            }else if (bug){
                rez += k*1LL;
                bug = 0;
            }
        }else bug += dr - st;
        i++;
        st = dr;
    }

    if (bug) rez += k;

    g << rez << "\n";

}

int main(){

    rezolva();

    f.close();
    g.close();

    return 0;

}