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

const int N = 5 * 1000 * 1000 + 17;

int nies = 1000 * 1000 * 1000 + 17;

int dp[N];
bool tab[N];
int pre[N];

int n, m, k, l;


int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  cin >> n >> m >> k >> l;

  for(int i = 0; i < m; i++){
    int pom;
    cin >> pom;
    tab[pom] = true;
  }

  // cout << "PRE \n";
  for(int i = 1; i <= n + k + l + 1; i++){
    pre[i] = pre[i-1] + tab[i - 1];
    dp[i] = nies;
    // cout << pre[i] << " ";
  }
  // cout << "\n";
  dp[1] = 0;


  for(int i = 1; i <= n + k + l + 1; i++){
    if( pre[i+k+l] - pre[i+k] == 0 ){
      dp[i+k+l] = dp[i] + 1;
    }
    if(tab[i] == 0){
      dp[i+1] = min(dp[i+1], dp[i]) ;
    }
  }

  // cout << "DP\n";
  // for(int i = 1; i <= n + k + l; i++){
  //   cout << dp[i] << " ";
  // }
  // cout << "\n";

  int odp = nies;
  for(int i = n + 1; i <= n + k + l + 1; i++){
    odp = min(odp, dp[i]);
  }
  if(odp >= nies){
   cout <<  -1 << "\n";
   return 0;
  }
  cout << odp << "\n";
  return 0;
}


 /*

13 4 3 3
3 7 8 13
3


7 4 1 2
2 5 6 7
-1

10 2 3 1
4 7
2


 */