import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, 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; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

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)); }


// floor(a / b)
Int divFloor(Int)(Int a, Int b) {
  return a / b - (((a ^ b) < 0 && a % b != 0) ? 1 : 0);
}

// ceil(a / b)
Int divCeil(Int)(Int a, Int b) {
  return a / b + (((a ^ b) > 0 && a % b != 0) ? 1 : 0);
}


/*
  A[i] = K Q[i] + R[i]
  # op on i: K x[i] + R[i]  (0 <= x[i] <= Q[i])
  K x[i] + R[i] <= (\sum[j] (K x[j] + R[j])) / K
  
  fix s := \sum[j] x[j]
  K x[i] <= s + sumR/K - R[i]
*/
long solve(int N, int K, long[] A) {
  auto Q = new long[N];
  auto R = new long[N];
  foreach (i; 0 .. N) {
    Q[i] = A[i] / K;
    R[i] = A[i] % K;
  }
  const sumR = R.sum;
  if (sumR % K != 0) return -1;
  const base = sumR / K;
  
  // p s + q
  alias Entry = Tuple!(long, "s", long, "p", long, "q");
  Entry[] es;
  es ~= Entry(0, 0, 0);
  foreach (i; 0 .. N) {
    // s + r  vs  0, K Q[i]
    const r = base - R[i];
    const s0 = max(0 - r, 0);
    const s1 = max(K * Q[i] - r, 0);
    es ~= Entry(s0, 1, r);
    es ~= Entry(s1, 0 - 1, K * Q[i] - r);
  }
  es.sort;
  const esLen = cast(int)(es.length);
  long p, q;
  foreach (j; 0 .. esLen) {
    long l = es[j].s;
    long r = (j + 1 == esLen) ? long.max : es[j + 1].s;
    // K s <= p s + q
    p += es[j].p;
    q += es[j].q;
    if (K < p) {
      chmax(l, divCeil(-q, p - K));
    } else if (K == p) {
      if (0 > q) continue;
    } else {
      chmin(r, divFloor(q, K - p));
    }
    if (l < r) {
      return l + base;
    }
  }
  return -1;
}

void main() {
  try {
    for (; ; ) {
      const numCases = readInt;
      foreach (caseId; 0 .. numCases) {
        const N = readInt;
        const K = readInt;
        auto A = new long[N];
        foreach (i; 0 .. N) {
          A[i] = readLong;
        }
        
        const ans = solve(N, K, A);
        writeln(ans);
      }
    }
  } catch (EOFException e) {
  }
}