#include <bits/stdc++.h>
using namespace std;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define RFOR(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define ALL(x) (x).begin(), (x).end()
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define RITR(x,c) for(__typeof(c.rbegin()) x=c.rbegin();x!=c.rend();x++)
#define BIT(n) (1LL<<(n))
#define SZ(x) ((int)(x).size())
typedef long long ll;
// -------------------------------------

int N, L;
int X[100101];
int R[100101];

int main() {
  scanf("%d", &N);
  scanf("%d", &L);
  REP(i, N-1) scanf("%d", X+i+1);
  R[0] = X[0] = 0;

  /*
  puts("----");
  REP(i, N) {
    _P("X[%d]=%d\n", i, X[i]);
  }
  */

  REP(i, N-1) {
    int d = abs(X[i] - X[i+1]);
    if (d >= L) {
      puts("-1");
      return 0;
    }
    R[i+1] = R[i] + X[i+1];
  }
  /*
  REP(i, N) {
    _P("R[%d]=%d\n", i, R[i]);
  }
  */

  int ans = 0;
  FOR(i, 1, N) {
    double j = 1.0 * (R[N-1] - R[i-1]) / (N-i);
    //_P("j=%f\n", j);
    if (abs(X[i-1] - j) * 2 >= L ||
        abs(X[i] - j) * 2 >= L) ans++;
  }

  cout << ans << endl;

  return 0;
}