#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MN = 100010;
int N, K, T;
int X[MN];
bool check(int x) {
vector<ll> le, ri;
for(int i = 0; i < K; i++) {
ll d = 1LL * T * x - (X[i + 1] - X[i]);
le.push_back(d);
}
reverse(le.begin(), le.end());
for(int i = K; i < N - 1; i++) {
ll d = 1LL * T * x - (X[i + 1] - X[i]);
ri.push_back(d);
}
vector<ll> tmp;
ll sum = 0;
for(int i = 0; i < le.size(); i++) {
sum += le[i];
if(i == (int)le.size() - 1 || ((le[i] >= 0) ^ (le[i + 1] >= 0))) tmp.push_back(sum), sum = 0;
}
le = tmp;
tmp.clear();
sum = 0;
for(int i = 0; i < ri.size(); i++) {
sum += ri[i];
if(i == (int)ri.size() - 1 || ((ri[i] >= 0) ^ (ri[i + 1] >= 0))) tmp.push_back(sum), sum = 0;
}
ri = tmp;
vector<pair<ll, ll> > L, R;
ll init = 0;
if(le.size() && le[0] >= 0) init += le[0], le.erase(le.begin());
if(ri.size() && ri[0] >= 0) init += ri[0], ri.erase(ri.begin());
if(le.size() % 2) le.push_back(0);
if(ri.size() % 2) ri.push_back(0);
for(int i = 0; i < le.size(); i += 2) {
ll n = -le[i];
ll p = le[i + 1];
L.push_back({ n, n - p });
while(L.size() >= 2 && L.back().second <= 0 && L[ (int)L.size() - 2 ].second > 0) {
pair<ll, ll> d = L.back(); L.pop_back();
pair<ll, ll> t = L.back(); L.pop_back();
t.first = max(t.first, t.second + d.first);
t.second += d.second;
L.push_back(t);
}
}
for(int i = 0; i < ri.size(); i += 2) {
ll n = -ri[i];
ll p = ri[i + 1];
R.push_back({ n, n - p });
while(R.size() >= 2 && R.back().second <= 0 && R[ (int)R.size() - 2 ].second > 0) {
pair<ll, ll> d = R.back(); R.pop_back();
pair<ll, ll> t = R.back(); R.pop_back();
t.first = max(t.first, t.second + d.first);
t.second += d.second;
R.push_back(t);
}
}
reverse(L.begin(), L.end());
reverse(R.begin(), R.end());
while(1) {
if(L.size() && L.back().second <= 0 && init >= L.back().first) {
init -= L.back().second, L.pop_back();
continue;
}
if(R.size() && R.back().second <= 0 && init >= R.back().first) {
init -= R.back().second, R.pop_back();
continue;
}
break;
}
if(L.size() && L.back().second <= 0) return false;
if(R.size() && R.back().second <= 0) return false;
reverse(L.begin(), L.end());
reverse(R.begin(), R.end());
vector<ll> psum1(L.size());
vector<ll> psum2(R.size());
for(int i = 0; i < L.size(); i++) {
psum1[i] = L[i].second;
if(i) psum1[i] += psum1[i - 1];
}
for(int i = 0; i < R.size(); i++) {
psum2[i] = R[i].second;
if(i) psum2[i] += psum2[i - 1];
}
if(init < (psum1.size()? psum1.back() : 0) + (psum2.size()? psum2.back() : 0)) return false;
vector<int> pnt1(L.size());
vector<int> pnt2(R.size());
for(int i = 0; i < L.size(); i++) {
ll rem = i? init - psum1[i - 1] : init;
if(rem < L[i].first) return false;
rem -= L[i].first;
int s = 0, e = (int)psum2.size() - 1, p = -1;
while(s <= e) {
int m = (s + e)>>1;
if(psum2[m] <= rem) {
p = m;
s = m + 1;
}
else e = m - 1;
}
pnt1[i] = p;
}
for(int i = 0; i < R.size(); i++) {
ll rem = i? init - psum2[i - 1] : init;
if(rem < R[i].first) return false;
rem -= R[i].first;
int s = 0, e = (int)psum1.size() - 1, p = -1;
while(s <= e) {
int m = (s + e)>>1;
if(psum1[m] <= rem) {
p = m;
s = m + 1;
}
else e = m - 1;
}
pnt2[i] = p;
}
int pos = (int)R.size() - 1;
int mn1 = 1e9;
int mn2 = 1e9;
for(int i = (int)L.size() - 1; i >= 0; i--) {
mn1 = min(mn1, pnt1[i]);
while(pos >= 0 && pos > mn1) {
mn2 = min(mn2, pnt2[pos]);
pos--;
}
if(mn2 < i) return false;
}
return true;
}
int main() {
scanf("%d %d %d", &N, &K, &T);
K--;
for(int i = 0; i < N; i++) {
scanf("%d", &X[i]);
}
int s = 0, e = 1e9 / T + 1, p = -1;
while(s <= e) {
int m = (s + e)>>1;
if(check(2 * m)) {
p = m;
e = m - 1;
}
else s = m + 1;
}
printf("%d", p);
}
