import java.util.Scanner;
class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();
		int L = sc.nextInt();
		int[] minX = new int[N];// 上から順に0,1,2,...と振る
		int[] maxX = new int[N];
		double[] G = new double[N-1];
		int count_glue = 0;

		boolean flag = false;
		minX[N-1] = 0;
		maxX[N-1] = L;
		for(int i=N-2;i>=0;i--){
			minX[i] = sc.nextInt();
			maxX[i] = minX[i]+L;
			if(minX[i+1] >= maxX[i] || maxX[i+1] <= minX[i])flag = true;
		}
		sc.close();

		if(flag)System.out.println(-1);
		else{
			double sum = 0;
			for(int i=0;i<N-1;i++){
				sum += minX[i];
				G[i] = sum/(i+1) + L/2.0;

				boolean notfixed = G[i] <= minX[i+1] || G[i] >= maxX[i+1]
						|| G[i] <= minX[i] || G[i] >= maxX[i];
				if(notfixed){
					count_glue++;
				}
			}
			System.out.println(count_glue);
		}
	}

}