CF D. One-Dimensional Battle Ships

#include<iostream>
#include<cstdio>
#include<cstring>
#include<set>
using namespace std;
int L,N,a,M;
int main()
{
    set<int> Pos;
    set<int>::iterator it;
    cin >> L >> N >> a;
    {
        Pos.clear();
        Pos.insert(0),Pos.insert(L+1);
        int ans =1 + (L - a) / (a + 1);
        cin >> M;
        for(int i = 1; i <= M; ++i)
        {
            int x;
            cin >> x;
            it = Pos.lower_bound(x);
            int r = *it - 1;
            int l = *(--it) + 1;
            int len = r - l + 1;
            int Sum = 0, SumR = 0, SumL = 0;
            if(len >= a)
                Sum = 1 + (len - a) / (a + 1);
            if(x - l >= a)
                SumL = 1 + (x - l - a) / (a + 1);
            if(r - x >= a)
                SumR = 1 + (r - x - a) / (a + 1);
            ans = ans - Sum + SumL + SumR;
            if(ans < N) {
                cout << i << endl;
                return 0;
            }
            Pos.insert(x);
        }
        cout << "-1\n";
    }
    return 0;
 }