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

int main() {
  long long a,b,c,d;
  cin >> a >> b >> c >> d;
  long long n = 0;
  long long mizu = a;
  long long aka = 0;
  if (b >= c*d){
    cout << -1 << endl;
  }
  
  else {
    while (mizu > aka * d) {
    mizu += b;
    aka += c;
    n++;
    }
    cout << n << endl;
  }
}
#include <bits/stdc++.h>
#define M0D 1000000007
#define rep(i, n) for(int i=0; i<n; i++)
using namespace std;

int main(){

  int n,k,m,ans;
  cin >> n >> k >> m;
  int a[n],sum=0;
  rep(i,n-1){
    cin >> a[i];
    sum += a[i];
  }
  ans = n*m - sum;
  if(ans>k) cout << "-1" << endl;
  else if(ans<0) cout << "0" << endl;
  else cout << ans << endl;

  return 0;
}
