1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4
 5 #define LL long long
 6 #define N 2010
 7 #define P 1000000007LL
 8
 9 using namespace std;
10
11 int n;
12 LL w[N],S[N],S2[N],f[N][3][N];
13
14 LL sum(LL S[],int l,int r)
15 {
16     if(l>r) return 0LL;
17     LL ans = S[r]+P-S[l-1];
18     if(ans>=P) ans-=P;
19     return ans;
20 }
21
22 int main()
23 {
24 //    freopen("test.txt","r",stdin);
25     while(~scanf("%d",&n))
26     {
27         for(int i=1;i<=n;i++) scanf("%lld",&w[i]);
28         for(int i=0;i<=n-2;i++)
29             for(int k=1;k<=n;k++)
30                 f[i][0][k]=0, f[i][1][k]=0, f[i][2][k]=0;
31         for(int x1=1;x1<=n;x1++)
32             for(int x2=x1;x2<=n;x2++) f[0][2][x2]++;
33         for(int x1=1;x1<=n;x1++) f[0][1][x1]=1;
34         for(int i=1;i<=n-2;i++)
35         {
36             for(int k=1;k<=n;k++)
37             {
38                 S[k]  = S[k-1] +f[i-1][0][k];
39                 S2[k] = S2[k-1]+f[i-1][0][k]*(k-1)*(k-1);
40                 S2[k] += f[i-1][1][k]*(k-1);
41                 S2[k] += f[i-1][2][k];
42             }
43             for(int k=1;k<=n;k++)
44             {
45                 f[i][2][k] += sum(S2,1,k-1);
46                 f[i][1][k] += f[i-1][2][k];
47                 f[i][0][k] += f[i-1][1][k];
48                 f[i][2][k] += f[i-1][0][k]*(k-1)*(k-1);
49                 f[i][2][k] += f[i-1][1][k]*(k-1);
50                 f[i][2][k] += f[i-1][2][k];
51                 f[i][0][k] += sum(S,k+1,n);
52                 f[i][1][k] += sum(S,k+1,n)*k;
53                 f[i][2][k] += sum(S,k+1,n)*k*k;
54                 f[i][0][k] = f[i][0][k]%P * w[k]%P;
55                 f[i][1][k] = f[i][1][k]%P * w[k]%P;
56                 f[i][2][k] = f[i][2][k]%P * w[k]%P;
57             }
58         }
59         LL ans=0;
60         for(int k=1;k<=n;k++)
61         {
62             ans += f[n-2][0][k]*(k-1)*(k-1);
63             ans += f[n-2][1][k]*(k-1);
64             ans += f[n-2][2][k];
65             ans %= P;
66         }
67         cout << ans << endl;
68     }
69     return 0;
70 }