Codeforces Round #Pi (Div. 2)

#include <bits/stdc++.h>
using namespace std;
const int maxn = 200005;
int shot1[maxn], shot2[maxn];
int n, m, a, k;
int solve (int x)
{
for (int i=1; i<=x; i++)
shot2[i] = shot1[i];
int ans = 0;
shot2[0] = 0;
shot2[x+1] = n+1;
sort (shot2, shot2+x+2);
for (int i=1; i<=x+1; i++)
ans += (shot2[i] - shot2[i-1]) / (a + 1);
return ans;
}
int main ()
{
while (scanf ("%d %d %d", &n, &k, &a) != EOF)
{
scanf ("%d", &m);
for (int i=1; i<=m; i++)
scanf ("%d", &shot1[i]);
int low = 1, high = m, mid, res = n + 1;
while (low <= high)
{
mid = (low + high) / 2;
int ans = solve (mid);

if (ans >= k)

low = mid + 1;
else
{
high = mid - 1;
res = min (res, mid);
}
}
if (res == n+1)
res = -1;
printf ("%d\n", res);
}
return 0;
}