import std.conv, std.functional, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N;
long L;
long[] X;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      L = readLong() * 2;
      X = new long[N];
      foreach (i; 1 .. N) {
        X[i] = readLong() * 2;
      }
      
      int ans;
      long sum;
      foreach_reverse (i; 0 .. N - 1) {
        sum += X[i + 1];
        const a = max(X[i], X[i + 1]);
        const b = min(X[i] + L, X[i + 1] + L);
        if (a < b) {
          // a < sum / (N - 1 - i) + L / 2 < b
          if ((a - L / 2) * (N - 1 - i) < sum && sum < (b - L / 2) * (N - 1 - i)) {
            // tenuki
          } else {
            ++ans;
          }
        } else {
          ans = -1;
          break;
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}