ll@n,@l,@k,a[n+2],c[],h=0,b=l;
rd(((a+1))(n));
a[0]=0;
a[n+1]=l;
while(h+1<b){
	ll m=h+b>>1;
	ll y=0;
	c[0]=-3d5;
	rep(x,n+2){
		while(a[y]-a[x]<m){
			if(++y==n+2){
				b=m;
				goto next;
			}
			c[y]=c[y-1]+1;
		}
		c[y]>?=x;
		if(c[x]+n-y>k-3){
			h=m;
			goto next;
		}
	}
	next:;
}
wt(h);