#include <bits/stdc++.h>
#define inf 2e9
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair <int, int> pii;
const int N = 1000 + 3;
int n, m, k, a[N], ans, sz = k * 2 + 1;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
#ifdef LOCAL
freopen("input.txt", "r", stdin);
#endif // LOCAL
cin >> n >> m >> k;
sz = k * 2 + 1;
for (int i = 0; i < m; i++)
cin >> a[i];
for (int i = 1; i < m; i++){
ans += max(0, (a[i] - a[i - 1] - 1 - 2 * k + sz - 1) / sz);
}
ans += max(0,(a[0] - k + sz - 1) / sz);
ans += max(0,(n - a[m-1] - k + sz - 1) / sz);
cout << ans;
}
