Language: C++

#include <bits/stdc++.h>
using namespace std;

#define li          long long int
#define rep(i,to)   for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)    for(li i=(li)(start);i<((li)(to));i++)
#define pb          push_back
#define sz(v)       ((li)(v).size())
#define bgn(v)      ((v).begin())
#define eend(v)     ((v).end())
#define allof(v)    (v).begin(), (v).end()
#define dodp(v,n)       memset(v,(li)n,sizeof(v))
#define bit(n)      (1ll<<(li)(n))
#define mp(a,b)     make_pair(a,b)
#define rin rep(i,n)
#define rjm rep(j,m)


#define DBGP 1


#define idp if(DBGP)
#define F first
#define S second
#define p2(a,b)     idp cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)       idp cout<<a<<"\t"<<b<<"\t"<<c<<endl
#define p4(a,b,c,d)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<endl
#define p5(a,b,c,d,e)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<endl
#define p6(a,b,c,d,e,f)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<endl
#define p7(a,b,c,d,e,f,g)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<endl
#define p8(a,b,c,d,e,f,g,h)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<endl
#define p9(a,b,c,d,e,f,g,h,i)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<endl
#define p10(a,b,c,d,e,f,g,h,i,j)        idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<"\t"<<j<<endl
#define foreach(it,v)   for(__typeof((v).begin()) it=(v).begin(); it!=(v).end(); ++it)
#define p2p(x)      idp p2((x).F, (x).S)
#define dump(x,n)   idp{rep(i,n){cout<<x[i]<<" ";}puts("");}
#define dump2(x,n)  idp{rep(i,n){cout<<"["<<x[i].F<<" , "<<x[i].S<<"] ";}puts("");}
#define dumpi(x)    idp{foreach(it, x){cout<<(*it)<<" ";}puts("");}
#define dumpi2(x)   idp{foreach(it, x){cout<<"["<<(it)->F<<" , "<<(it)->S<<"] ";}puts("");}

#define read2d(a,w,h)   rep(i,h)rep(j,w)cin>>a[i][j]
#define dump2d(a,w,h)   rep(i,h){rep(j,w)cout<<a[i][j]<<" ";puts("");}


#define EPS 1e-10
#define ETOL 1e-8
#define MOD 100000009

li n;
li a[100100];
li m, v, p;

inline bool check(li pos) {
    if (pos >= n - p) {
        return true;
    }
    vector<li> aa(n);
    rin{
        aa[i] = a[i];
    }
    li rest = v;
    if (pos >= v - 1) {
        rep(i, v - 1) {
            aa[i] += m;
        }
        aa[pos] += m;
        rest = 0;
    } else {
        rep(i, pos + 1) {
            aa[i] += m;
        }
        if (aa[i] < aa[n - p]) {
            return false;
        }
        rest -= pos + 1;
        if (rest <= p - 1) {
            rep(i, rest) {
                aa[n - 1 - i] += m;
            }
        } else {
            rep(i, p - 1) {
                aa[n - 1 - i] += m;
            }
            rest -= p - 1;
            vector<li> aaa;
            repp(i, pos + 1, n - p + 1) {
                aaa.pb(aa[pos] - aa[i]);
                if (aaa.back() < 0) {
                    return false;
                }
            }
            priority_queue<li> pq;
            rep(i, rest) {
                pq.push(m);
            }
            for (auto v : aaa) {
                if (pq.empty()) {
                    break;
                }
                li now = pq.top();
                pq.pop();
                now -= min(now, v);
                if (now > 0) {
                    pq.push(now);
                }
            }
            return pq.empty();
        }
    }
    //cout << pos << " : "; dumpi(aa);
    li orig = aa[pos];
    sort(allof(aa));
    auto it = upper_bound(allof(aa), orig);
    it--;
    auto idx = it - aa.begin();
    //p5(pos, orig, *it, idx, (idx >= n - p));
    return idx >= n - p;
}

int main() {
    cin >> n >> m >> v >> p;
    rin{
        cin >> a[i];
    }
    sort(a, a + n);
    if (check(0)) {
        cout << n << endl;
        return 0;
    }
    //dump(a, n);
    li l = 0;
    li r = n - 1;
    while (l + 1 < r) {
        li med = (l + r) / 2;
        if (check(med)) {
            r = med;
        } else {
            l = med;
        }
    }
    cout << n - r << endl;

    return 0;
}

Compiler output:
a.cc: In function 'bool check(long long int)':
a.cc:72:16: error: 'i' was not declared in this scope
   72 |         if (aa[i] < aa[n - p]) {
      |                ^