#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int > pp;
typedef long long ll;
int const N=1e4+2,oo=1e9,M=1e6;
ll const OO=1e18;
double const eps=1e-6,PI=acos(-1);
int mod=M+7;
int n,aa[N],dp[2][N][2];
int rec(int idx,int mx,int l){
int &ret=dp[idx][mx][l];
if(ret!=-1)return ret;
if(idx==n-1)return ret=1;
if(idx==0){
if(aa[1]>1)return ret=(rec(idx+1,2,0)+rec(idx+1,1,1))%mod;
return ret=rec(idx+1,1,0);
}
if(l)return ret=(rec(idx+1,mx,l)*1LL*mx+rec(idx+1,mx+1,l))%mod;
else{
if(aa[idx+1]>mx)ret=(rec(idx+1,mx,1)*1LL*mx+rec(idx+1,aa[idx+1],0))%mod;
else ret=(rec(idx+1,mx,1)*1LL*(aa[idx+1]-1)+rec(idx+1,mx,0))%mod;
return ret;
}
return ret;
}
int main(){
scanf("%d",&n);
for(int i=0;i<n;i++)scanf("%d",&aa[i]);
/* memset(dp,-1,sizeof dp);
cout<<rec(0,0,0)<<endl;*/
int f=0,an=0;
for(int i=1;i<=n;i++)dp[f][i][0]=dp[f][i][1]=1;
for(int i=n-2;i>-1;i--){
f=1-f;
for(int j=1;j<=n;j++){
dp[f][j][1]=(dp[!f][j][1]*1LL*j+dp[!f][j+1][1])%mod;
if(aa[i+1]>j)dp[f][j][0]=(dp[!f][j][1]*1LL*j+(aa[i+1]==j+1)*dp[!f][aa[i+1]][0])%mod;
else dp[f][j][0]=(dp[!f][j][1]*1LL*(aa[i+1]-1)+dp[!f][j][0])%mod;
}
}
printf("%d\n",dp[f][1][0]);
// printf("%d\n",an);
return 0;
}
