Problem:
Score : 800 points

Problem Statement

There are N children, numbered 1,2,\ldots,N.
In the next K days, we will give them some cookies.
In the i-th day, we will choose a_i children among the N with equal probability, and give one cookie to each child chosen. (We make these K choices independently.)

Let us define the happiness of the children as c_1 \times c_2 \times \ldots \times c_N, where c_i is the number of cookies received by Child i in the K days.
Find the expected happiness multiplied by \binom{N}{a_1} \times \binom{N}{a_2} \times \ldots \times \binom{N}{a_K} (we can show that this value is an integer), modulo (10^{9}+7).

Notes

\binom{n}{k} denotes the number of possible choices of k objects out of given distinct n objects, disregarding order.

Constraints

1 \leq N \leq 1000

1 \leq K \leq 20

1 \leq a_i \leq N

Input

Input is given from Standard Input in the following format:

N K
a_1 a_2 \ldots a_K

Output

Print the answer.

Sample Input 1

3 2
3 2

Sample Output 1

12

On the first day, all the children 1, 2, and 3 receive one cookie.

On the second day, two out of the three children 1, 2, and 3 receive one cookie.

Their happiness is 4 in any case, so the expected happiness is 4. Print this value multiplied by \binom{3}{3} \times \binom{3}{2}, which is 12.

Sample Input 2

856 16
399 263 665 432 206 61 784 548 422 313 848 478 827 26 398 63

Sample Output 2

337587117

Find the expected value multiplied by \binom{N}{a_1} \times \binom{N}{a_2} \times \ldots \times \binom{N}{a_K}, modulo (10^9+7).

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

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

#define int long long

#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second

using vint=vector<int>;
using pint=pair<int,int>;
using vpint=vector<pint>;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}

template<class A,class B>
ostream& operator<<(ostream& ost,const pair<A,B>&p){
    ost<<"{"<<p.first<<","<<p.second<<"}";
    return ost;
}

template<class T>
ostream& operator<<(ostream& ost,const vector<T>&v){
    ost<<"{";
    for(int i=0;i<v.size();i++){
        if(i)ost<<",";
        ost<<v[i];
    }
    ost<<"}";
    return ost;
}

inline int topbit(unsigned long long x){
    return x?63-__builtin_clzll(x):-1;
}

inline int popcount(unsigned long long x){
    return __builtin_popcountll(x);
}

inline int parity(unsigned long long x){
    return __builtin_parity(x);
}

template<uint32_t mod>
struct ModInt{
    uint32_t a;
    ModInt& s(uint32_t vv){
        a=vv<mod?vv:vv-mod;
        return *this;
    }

    ModInt(int64_t x=0){s(x%mod+mod);}

    ModInt& operator+=(const ModInt &x){return s(a+x.a);}
    ModInt& operator-=(const ModInt &x){return s(a+mod-x.a);}
    ModInt& operator*=(const ModInt &x){
        a=uint64_t(a)*x.a%mod;
        return *this;
    }
    ModInt& operator/=(const ModInt &x){
        *this*=x.inv();
        return *this;
    }

    ModInt operator+(const ModInt &x)const{return ModInt(*this)+=x;}
    ModInt operator-(const ModInt &x)const{return ModInt(*this)-=x;}
    ModInt operator*(const ModInt &x)const{return ModInt(*this)*=x;}
    ModInt operator/(const ModInt &x)const{return ModInt(*this)/=x;}
    bool operator==(const ModInt &x)const{return a==x.a;}
    bool operator!=(const ModInt &x)const{return a!=x.a;}
    bool operator<(const ModInt &x)const{return a<x.a;}

    ModInt operator-()const{return ModInt()-*this;}
    ModInt pow(int64_t n)const{
        ModInt res(1),x(*this);
        while(n){
            if(n&1)res*=x;
            x*=x;
            n>>=1;
        }
        return res;
    }

    ModInt inv()const{return pow(mod-2);}
};

template<uint32_t mod>
istream& operator>>(istream& in,ModInt<mod>& a){
    return (in>>a.a);
}
template<uint32_t mod>
ostream& operator<<(ostream& out,const ModInt<mod>& a){
    return (out<<a.a);
}
using mint=ModInt<1000000007>;

template<class Mint,int32_t lg>
struct ModIntTable{
    int N;
    vector<Mint>facts,finvs,invs;
    ModIntTable():N(1<<lg),facts(N),finvs(N),invs(N){
        const uint32_t mod=Mint(-1).a+1;
        invs[1]=1;
        for(int i=2;i<N;i++)invs[i]=invs[mod%i]*(mod-mod/i);

        facts[0]=1;
        finvs[0]=1;
        for(int i=1;i<N;i++){
            facts[i]=facts[i-1]*i;
            finvs[i]=finvs[i-1]*invs[i];
        }
    }
    inline Mint fact(int n)const{return facts[n];}
    inline Mint finv(int n)const{return finvs[n];}
    inline Mint inv(int n)const{return invs[n];}
    inline Mint binom(int n,int k)const{
        if(n<0||k<0||k>n)return 0;
        return facts[n]*finvs[k]*finvs[n-k];
    }
    inline Mint perm(int n,int k)const{
        if(n<0||k<0||k>n)return 0;
        return facts[n]*finvs[n-k];
    }
    inline Mint catalan(int n){
        return facts[2*n]*finvs[n+1]*finvs[n];
    }
};
ModIntTable<mint,20>mt;

int N,K;
int A[22];

mint dp[22][1111];

signed main(){
    cin>>N>>K;
    rep(i,N)cin>>A[i];

    dp[0][0]=1;
    rep(k,K){
        for(int i=0;i<=N;i++){
            for(int j=0;j<=A[k]&&i+j<=N;j++){
                dp[k+1][i+j]+=dp[k][i]*mt.binom(N-i,j)*mt.binom(N-j,A[k]-j);
            }
        }
    }

    cout<<dp[K][N]<<endl;
    return 0;
}