import java.util.*;
public class d {
	Scanner in=new Scanner(System.in);
	public static void main(String[] args) {
		new d().go();

	}
	private void go() {
		OUT:
		while(true){
			int n=in.nextInt(),d=in.nextInt();
			if(n==0)break;
			int[] heights=new int[n],pointin=new int[n],pointout=new int[n];
			for(int i=0;i<n;i++)heights[i]=in.nextInt();
			int[] sorted=Arrays.copyOf(heights, n);
			Arrays.sort(sorted);
			for(int i=0;i<n;i++)pointin[i]=indexof(heights,sorted[i]);
			if(pointin[0]>pointin[n-1])for(int i=0;i<n/2;i++){//reverse so taller tree is always to the right
				int temp=pointin[i];
				pointin[i]=pointin[n-i-1];
				pointin[n-1-i]=temp;
			}
			for(int i=0;i<n-1;i++)if(Math.abs(pointin[i]-pointin[i+1])>d){
				System.out.println(-1);
				continue OUT;
			}
			for(int i=0;i<n;i++)pointout[pointin[i]]=i;
			for(int i=0;i<n;i++)heights[i]=i;//repurpose heights as physical location
			for(int on=pointin[0]+1;on<n;on++){
				int min=Integer.MAX_VALUE,mini=0;
				for(int i=on;i<n;i++){
					if(pointout[i]!=0&&pointin[pointout[i]-1]<on&&d-(heights[i]-heights[pointin[pointout[i]-1]])<min){
						min=d-(heights[i]-heights[pointin[pointout[i]-1]]);
						mini=i;
					}
					if(pointout[i]!=n-1&&pointin[pointout[i]+1]<on&&d-(heights[i]-heights[pointin[pointout[i]+1]])<min){
						min=d-(heights[i]-heights[pointin[pointout[i]+1]]);
						mini=i;
					}
				}
				for(int j=on;j<=mini;j++)heights[j]+=min;
				on=mini;
			}
			System.out.println(Math.abs(heights[pointin[0]]-heights[pointin[n-1]]));
		}
	}
	private int indexof(int[] a,int b){
		for(int i=0;i<a.length;i++)if(a[i]==b)return i;
		return -1;
	}
}
//The Ninja Way
//greedy
//dp-ish