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

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;

#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define asd cout << "ok" << endl;
#define trav(i, n) for(int i= 0; i <n; i++)


const char nl = '\n';
const int maxN = 2e5 + 4;
int main(){
  ios_base::sync_with_stdio(0);cin.tie(0);
   int a, b, c ,d; cin >> a >> b >> c >> d;
   int ans = 0;
   if(c*d <= b){
    cout << -1;
    return 0;
   }
   cout << (a -1 + d*c -b)/ (d*c - b) << nl;
}

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

int main(){
  int a,s,d;
  int g,h=0;
  cin >> a>>s>>d;
  vector<int>f(a);
  for(int i=0;i<a-1;i++){
    cin>>g;
    h=h+g;
  }
  g=d*a-h;
  if(d*a-h<0)g=0;
  if(d*a-h<=s)  cout <<g;
  else if(d*a-h>s)  cout <<-1;

}