#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

char*mmap();
#define rd(v) long v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;}
#define rd_signed(v) long v=0;{int _c,_s;_s=*rp=='-'&&++rp;while(_c=*rp++-48,_c>=0)v=v*10+_c;v=_s?-v:v;}
#define wt(v) {long _z=v;do*--wp=_z%10+48;while(_z/=10);}

#define rep3(v,s,e) for(long v=s;v<e;++v)
#define rrep3(v,s,e) for(long v=e;--v>=s;)
#define max(a,b) (a>=b?a:b)
#define min(a,b) (a<=b?a:b)
#define abs(a) ((a)<0?-(a):(a))

int x[100000];

main(){
	char*rp=mmap(0l,1l<<25,1,2,0,0ll);
	rd(n);
	rd(l);
	rep3(i,1,n){
		rd_signed(xi);
		xi<<=1;
		if(abs(xi-x[i-1])>=l<<1){
			write(1,"-1",2);
			_exit(0);
		}
		x[i]=xi;
	}
	long r=0;
	long w=0;
	long k=0;
	rrep3(i,1,n){
		++k;
		w+=x[i];
		r+=(max(x[i],x[i-1])-l)*k>=w||(min(x[i],x[i-1])+l)*k<=w;
	}
	char wbuf[64],*wp=wbuf+sizeof wbuf;
	wt(r);
	write(1,wp,wbuf+sizeof wbuf-wp);
	_exit(0);
}