Problem p02804 (C++)

Buggy submission (status: Runtime Error):
/*input
10 6
1000000000 1000000000 1000000000 1000000000 1000000000 0 0 0 0 0
*/
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
const ll maxn = 20 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
ll sub(ll a, ll b) {
  ll x = a - b;
  while (x < 0)
    x += MOD;
  while (x > MOD)
    x -= MOD;
  return x;
}
ll mult(ll a, ll b) { return (a * b) % MOD; }
ll mypow(ll a, ll b) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
ll fac[maxn];
ll arr[maxn];
int main() {
  int n, k;
  cin >> n >> k;
  fac[0] = 1;
  REP1(i, maxn - 1) fac[i] = (fac[i - 1] * (ll)i) % MOD;
  REP(i, n) cin >> arr[i];
  sort(arr, arr + n);
  ll big = 0, small = 0;
  REP(i, n) {
    ll x = arr[i];
    if (i >= k - 1)
      big += mult(x, mult(mult(fac[i], mypow(fac[k - 1], MOD - 2)),
                          mypow(fac[i - k + 1], MOD - 2)));
    if (i <= n - k)
      small += mult(x, mult(mult(fac[n - i - 1], mypow(fac[k - 1], MOD - 2)),
                            mypow(fac[n - i - 1 - k + 1], MOD - 2)));
  }
  cout << sub(big, small);
}

Fixed submission:
/*input
10 6
1000000000 1000000000 1000000000 1000000000 1000000000 0 0 0 0 0
*/
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<long long>, rb_tree_tag,
             tree_order_statistics_node_update>
    indexed_set;
#pragma GCC optimize("unroll-loops,no-stack-protector")
// order_of_key #of elements less than x
//  find_by_order kth element
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
#define f first
#define s second
#define pb push_back
#define REP(i, n) for (int i = 0; i < n; i++)
#define REP1(i, n) for (int i = 1; i <= n; i++)
#define FILL(n, x) memset(n, x, sizeof(n))
#define ALL(_a) _a.begin(), _a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
const ll maxn = 1e5 + 5;
const ll maxlg = __lg(maxn) + 2;
const ll INF64 = 4e18;
const int INF = 0x3f3f3f3f;
const ll MOD = 1e9 + 7;
const ld PI = acos(-1);
const ld eps = 1e-9;
#define lowb(x) x & (-x)
#define MNTO(x, y) x = min(x, (__typeof__(x))y)
#define MXTO(x, y) x = max(x, (__typeof__(x))y)
ll sub(ll a, ll b) {
  ll x = a - b;
  while (x < 0)
    x += MOD;
  while (x > MOD)
    x -= MOD;
  return x;
}
ll mult(ll a, ll b) { return (a * b) % MOD; }
ll mypow(ll a, ll b) {
  if (b <= 0)
    return 1;
  ll res = 1LL;
  while (b) {
    if (b & 1)
      res = (res * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
ll fac[maxn];
ll arr[maxn];
int main() {
  int n, k;
  cin >> n >> k;
  fac[0] = 1;
  REP1(i, maxn - 1) fac[i] = (fac[i - 1] * (ll)i) % MOD;
  REP(i, n) cin >> arr[i];
  sort(arr, arr + n);
  ll big = 0, small = 0;
  REP(i, n) {
    ll x = arr[i];
    if (i >= k - 1)
      big += mult(x, mult(mult(fac[i], mypow(fac[k - 1], MOD - 2)),
                          mypow(fac[i - k + 1], MOD - 2)));
    if (i <= n - k)
      small += mult(x, mult(mult(fac[n - i - 1], mypow(fac[k - 1], MOD - 2)),
                            mypow(fac[n - i - 1 - k + 1], MOD - 2)));
  }
  cout << sub(big, small);
}