#include <algorithm>
#include <cstdio>
using ll = long long;
struct ST_table
{
using func_T = int (*)(int, int);
int n, lg[100005], arr[25][100005];
func_T comp;
ST_table(int _n, func_T _comp) : n(_n), comp(_comp)
{
lg[0] = lg[1] = 0;
for (int i = 2; i <= n; i++)
lg[i] = lg[i >> 1] + 1;
for (int i = 0; i < n; i++)
arr[0][i] = i;
for (int i = 1; i <= lg[n]; i++)
{
for (int j = 0; j + (1 << i) <= n; j++)
arr[i][j] = comp(arr[i - 1][j], arr[i - 1][j + (1 << i - 1)]);
}
}
inline int operator ()(int l, int r)
{
int len = lg[r - l + 1];
return comp(arr[len][l], arr[len][r - (1 << len) + 1]);
}
};
int pos[100005], stk[100005], pre[100005], nxt[100005];
ll arr[100005];
inline int smaller(int x, int y) { return arr[x] < arr[y] ? x : y; }
inline int larger(int x, int y) { return arr[x] > arr[y] ? x : y; }
inline bool check(int n, int st, ll T)
{
for (int i = 0; i < n; i++)
arr[i] = pos[i] - T * i;
ST_table query_min(n, smaller), query_max(n, larger);
auto find_bigger = [&] (int *res)
{
int tp = 0;
stk[tp] = -1;
for (int i = 0; i < n; i++)
{
while (tp && arr[stk[tp]] <= arr[i])
tp--;
res[i] = stk[tp];
stk[++tp] = i;
}
std::reverse(arr, arr + n);
for (int i = 0; i < n; i++)
arr[i] *= -1;
};
find_bigger(pre);
find_bigger(nxt);
std::reverse(nxt, nxt + n);
for (int i = 0; i < n; i++)
nxt[i] = n - nxt[i] - 1;
int l = st, r = st;
while (l || r != n - 1)
{
auto try_l = [&] (int x)
{
if (x < 0 || arr[query_min(x, l - 1)] < arr[r])
return false;
l = x;
return true;
};
auto try_r = [&] (int y)
{
if (y >= n || arr[query_max(r + 1, y)] > arr[l])
return false;
r = y;
return true;
};
if (try_l(pre[l]) || try_r(nxt[r]))
continue;
if (!l)
return arr[query_max(r + 1, n - 1)] <= arr[l];
if (r == n - 1)
return arr[query_min(0, l - 1)] >= arr[r];
int x = query_min(0, l - 1), y = query_max(r + 1, n - 1);
if (arr[x] < arr[r] || arr[y] > arr[l])
return false;
int a = query_max(0, x), b = query_min(y, n - 1);
if (arr[a] >= arr[y])
l = a;
else if (arr[b] <= arr[x])
r = b;
else
return false;
}
return true;
}
int main()
{
// freopen("JOISC2017-D1T3.in", "r", stdin);
int n, st, T;
scanf("%d%d%d", &n, &st, &T);
for (int i = 0; i < n; i++)
scanf("%d", pos + i);
int l = 0, r = 1e9, res = -1;
while (l <= r)
{
int m = l + r >> 1;
if (check(n, st - 1, (ll)m * T * 2))
{
res = m;
r = m - 1;
}
else
l = m + 1;
}
printf("%d\n", res);
return 0;
}
