#include <bits/stdc++.h>
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
int n, d, m;
std::cin >> n >> d >> m;
std::vector<int> a(m + 1);
for(int i = 1; i <= m; i++) {
std::cin >> a[i];
}
std::sort(a.begin() + 1, a.end());
int l = 1, r = m, ans = - 1;
while(l <= r) {
int mid = (l + r) / 2;
std::set<int> st;
for(int i = 1; i <= mid; i++) {
st.insert(a[i]);
}
int ok = 1;
for(int i = mid + 1; i <= m; i++) {
auto it = st.begin();
if(*it < a[i]) {
int old = *it;
int New = std::max(old + 1, a[i]);
if(New > a[i] + d) {
ok = 0;
break;
}
st.insert(New);
st.erase(it);
}
else {
ok = 0;
break;
}
}
if(ok) {
r = mid - 1;
ans = mid;
}
else {
l = mid + 1;
}
}
std::cout << ans;
}
/**
**/
