#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int N; int L;
	while(~scanf("%d%d", &N, &L)) {
		vector<int> X(N);
		X[0] = 0;
		for(int i = 1; i < N; ++ i)
			scanf("%d", &X[i]);
		int ans = 0;
		ll num = 0; int den = 0;
		for(int i = N - 1; i >= 0; -- i) {
			if(i < N - 1) {
				if(num <= (ll)X[i] * den || (ll)(X[i] + L) * den <= num ||
				   num <= (ll)X[i + 1] * den || (ll)(X[i + 1] + L) * den <= num)
					++ ans;
				if(X[i] + L <= X[i + 1] || X[i + 1] + L <= X[i]) {
					ans = -1;
					break;
				}
			}
			num += X[i] * 2 + L;
			den += 2;
		}
		printf("%d\n", ans);
	}
	return 0;
}