STAIR.cpp

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

typedef long long ll;

const ll MOD = 1e9 + 7;

ll binpow(ll a, ll b) {
  if (b == 0) return 1;
  ll t = binpow(a, b / 2);
  t = (t * t) % MOD;
  if (b % 2) return (t * a) % MOD;
  return t;
}

ll fact(ll n) {
  if (n == 0) return 1;
  return (n * fact(n - 1)) % MOD;
}

ll modinv(ll n) {
  return binpow(n, MOD - 2);
}

ll comb(ll n, ll k) {
  return (fact(n) * modinv(fact(k)) % MOD * modinv(fact(n - k))) % MOD;
}

signed main() {
  cin.tie(0)->sync_with_stdio(0);
  ll n, k;
  cin >> n >> k;
  cout << comb(n + k - 1, k - 1) << '\n';
}