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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3. #define ll long long
  4. #define maxn (ll) (1e7+5)
  5. #define mod (ll)(1e9+7)
  6. ll f[maxn],sum,x,m;
  7. ll i,n;
  8. ll po(ll a,ll n){
  9.  ll  res=a,ans=1;
  10.   while(n){
  11.     if(n%2)
  12.         ans=ans*res%mod;
  13.         res=res*res%mod;
  14.         n/=2;
  15.   }
  16.    return ans;
  17. }
  18. ll C(ll n,ll k){
  19.    return f[n]*po(f[n-k],mod-2)%mod*po(f[k],mod-2)%mod;
  20. }
  21. int main(){
  22.    f[0]=1;
  23.    for(i=1;i<maxn;i++) f[i]=f[i-1]*i%mod;
  24.    cin>>m>>n;
  25.    for(i=0;i<n;i++){
  26.        cin>>x;
  27.        sum+=x;
  28.    }
  29.    if(m<sum) {cout<<0;return 0;}
  30.    ll tmp,j;
  31.    tmp=po(f[n-1],mod-2);
  32.    for(j=m-sum+1;j<=m-sum+n-1;j++) tmp=(tmp*j)%mod;
  33.    cout<<tmp;
  34.    return 0;
  35. }
  36.  
Success #stdin #stdout 0.06s 81592KB
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https://ideone.com/v7nDLj
language:
C++14 (gcc 8.3)
created:
1 year ago
visibility:
public

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