import java.util.Scanner;
import java.util.*;
  
public class Main {
	public static void main (String[] args) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();
		double s0 = scanner.nextDouble();
		double s = scanner.nextDouble();
		double [] A = new double[n];
		double [] B = new double[n];
		if(s0>=s)
			System.out.println(0);
		else if(n==0)
			System.out.println(-1);
		else
		{
		    for(int i=0; i<n; i++)
		    {
		        double a = scanner.nextDouble();
		        A[i]=a;
		        B[i]=A[i];
		    }
		    Arrays.sort(B);
		    int ans=0;
		    for(int i=n-1; i>=1; i--)
		    {
		        if(B[i]>=10.1)
		        {
		            if((int)(B[i]-5.1)+s0>=s){
		                ans++;
		                System.out.println(ans);
		                System.exit(0);
		            }
		            else
		            {
		                ans++;
		                B[i-1]+=(int)(B[i]-5.1);
		            }
		        }
		        else
		        {
		            break;
		        }
		    }
		    if((B[0]>=10.1)&&((int)(B[0]-5.1)+s0>=s))
		        System.out.println(ans+1);
		    else
		    {
		        Arrays.sort(A);
		        if(s0>=10.1)
		        {
		            A[n-1]+=(int)(s0-5.1);
		            s0-=(int)(s0-5.1);
						s0-=0.1;
		            ans=1;
		            for(int i=n-1; i>=1; i--)
		            {
		                if(A[i]>=10.1)
		                {
		                    if((int)(A[i]-5.1)+s0>=s){
		                        ans++;
		                        System.out.println(ans);
		                        System.exit(0);
		                    }
		                    else
		                    {   
		                        ans++;
		                        A[i-1]+=(int)(A[i]-5.1);
		                    }
		                }
		                else
		                {
		                    break;
		                }
		            }
		            if((A[0]>=10.1)&&((int)(A[0]-5.1)+s0>=s))
		                System.out.println(ans+1);
		            else
		                System.out.println(-1);
		        }
		        else
		        	System.out.println(-1);
		    }   
		}	
	}
}