#include <bits/stdc++.h>

#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(x) (x).begin(),(x).end()

using namespace std;

typedef long double ld;

const ld eps = 1e-9;

int main() {
  int N; ld L; cin >> N >> L;
  vector<int> X(N);
  REP(i,N-1) cin >> X[i+1];
  ld sum = 0, cnt = 0, res = 0;
  for (int i = N-1; i >= 1; --i) {
    sum += X[i] + L / 2; ++cnt;
    ld g = sum / cnt;
    if (g < X[i] + eps || X[i] + L < g + eps ||
        g < X[i-1] + eps || X[i-1] + L < g + eps) ++res;
  }
  REP(i,N-1) if (abs(X[i] - X[i+1]) > L - eps) res = -1;
  cout << res << endl;
  return 0;
}