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  1. #include <bits/stdc++.h>
  2.  
  3. #define el '\n'
  4. #define fi first
  5. #define sec second
  6. #define pb push_back
  7. #define ll long long
  8. #define sz(v) (int)(v).size()
  9. #define all(v) (v).begin(),(v).end()
  10. #define FOR(i, a, b) for(int i = (a), _b = b; i <= _b; i++)
  11. #define REP(i, a, b) for(int i = (a), _b = b; i >= _b; i--)
  12.  
  13. using namespace std;
  14.  
  15. const int MAX_N = 1e7;
  16. const int MOD = 1e9 + 7;
  17.  
  18. int fact[2 * MAX_N + 5], invfact[2 * MAX_N + 5];
  19. int n, m;
  20.  
  21. void Input(){
  22.     cin >> n >> m;
  23. }
  24.  
  25. ll bin_pow(ll a, int b){
  26.     ll res = 1;
  27.     while(b){
  28.         if(b & 1) res = (res * a) % MOD;
  29.         a = (a * a) % MOD;
  30.         b /= 2;
  31.     }
  32.     return res;
  33. }
  34.  
  35. ll C(int k, int n){
  36.     return 1LL * fact[n] * invfact[k] % MOD * invfact[n - k] % MOD;
  37. }
  38.  
  39. void Solve(){
  40.  
  41.     fact[0] = 1;
  42.     FOR(i, 1, n + m) fact[i] = (1LL * fact[i - 1] * i) % MOD;
  43.  
  44.  
  45.     invfact[n + m] = bin_pow(fact[n + m], MOD - 2);
  46.     REP(i, n + m - 1, 0) invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
  47.  
  48.     ll ans = 0;
  49.     FOR(k, 0, min(n, m)){
  50.         ans = (ans + (1LL * C(n - k, n + m - k) * C(m - k, m)) % MOD) % MOD;
  51.     }
  52.  
  53.     cout << ans;
  54. }
  55.  
  56. int main(){
  57.     ios_base::sync_with_stdio(0);
  58.     cin.tie(0);
  59.  
  60.     Input();
  61.     Solve();
  62. }
  63.  
Success #stdin #stdout 0.01s 5704KB
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https://ideone.com/9c3aXS
language:
C++14 (gcc 8.3)
created:
1 hour ago
visibility:
public

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