#include <bits/stdc++.h>
            #include<iostream>
            #include<cstdio>
            #include<vector>
            #include<queue>
            #include<map>
            #include<cstring>
            #include<string>
            #include <math.h>
            #include<algorithm>
        //    #include <boost/multiprecision/cpp_int.hpp>
            #include<functional>
     #define int long long
            #define mod  1000000007
            #define pa pair<int,int>
            #define ll long long
            #define pal pair<double,int>
            #define ppa pair<int,pa>
            #define ssa pair<string,int>
            #define  mp make_pair
            #define  pb push_back
            #define EPS (1e-10)
            #define equals(a,b) (fabs((a)-(b))<EPS)
     
            using namespace std;
     //priority_queue<int, vector<int>, greater<int> > que;
            class Point{
            	public:
            	double x,y;
            	Point(double x=0,double y=0):x(x),y(y) {}
            	Point operator + (Point p) {return Point(x+p.x,y+p.y);}
            	Point operator - (Point p) {return Point(x-p.x,y-p.y);}
            	Point operator * (double a) {return Point(x*a,y*a);}
            	Point operator / (double a) {return Point(x/a,y/a);}
            	double absv() {return sqrt(norm());}
            	double norm() {return x*x+y*y;}
            	bool operator < (const Point &p) const{
            		return x != p.x ? x<p.x: y<p.y;
            	}
            	bool operator == (const Point &p) const{
            		return fabs(x-p.x)<EPS && fabs(y-p.y)<EPS;
            	}
            };
            typedef Point Vector;
     
            struct Segment{
            Point p1,p2;
            };
     
        double hen(Vector a){
        if(fabs(a.x)<EPS && a.y>0) return acos(0);
        else if(fabs(a.x)<EPS && a.y<0) return 3*acos(0);
        else if(fabs(a.y)<EPS && a.x<0) return 2*acos(0);
        else if(fabs(a.y)<EPS && a.x>0) return 0.0;
        else if(a.y>0) return acos(a.x/a.absv());
        else return 2*acos(0)+acos(-a.x/a.absv());
     
        }
     
string itos( int i ) {
ostringstream s ;
s << i ;
return s.str() ;
}

int gcd(int v,int b){
	if(v>b) return gcd(b,v);
	if(v==b) return b;
	if(b%v==0) return v;
	return gcd(v,b%v);
}
            double dot(Vector a,Vector b){
            	return a.x*b.x+a.y*b.y;
            }
            double cross(Vector a,Vector b){
            	return a.x*b.y-a.y*b.x;
            }
        
double distans(double x1,double y1,double x2,double y2){
	double rr=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
	return sqrt(rr);
	
}
            //----------------kokomade tenpure------------
int n;
int a[100020],b[10];
vector<int>ve;

int rui[100020];


    signed  main(){
    	rui[0]=1;
    	for(int i=1;i<100020;i++){
    		rui[i]=rui[i-1]*i;
    		rui[i]%=mod;
    	}
 cin>>n;
   // 	if(n>10) return 0;
 //   	for(int i=0;i<n;i++)ve.pb(i);
    	
    	for(int i=0;i<n;i++) cin>>a[i];
    	a[0]=1;
    	for(int i=1;i<n;i++){
    		if(a[i]!=a[i-1]+1) a[i]=a[i-1]+2;
    	}
   // 	for(int i=0;i<n;i++) cout<<a[i]<<" ";
    //	cout<<endl;
    	int ans=1;
    	int w;
    	for(int i=1;i<n;i++){
    		if(a[i]>a[i-1]+2) a[i]=a[i-1]+2;
    		if(i==n-1) a[i]=a[i-1]+1;
    		if(a[i]==a[i-1]+2) continue;
    		
    		if(i==n-1){
    			w=n-ve.size();
    		//	ve.pb(n-ve.size());
    			break;
    		}
    	//	cout<<i+1<<" "<<ve.size()<<endl;
    		ve.pb(i+1-ve.size());
    		
    		a[i]=a[i-1];
    		
    	}
    //	for(int i=0;i<ve.size();i++) cout<<ve[i]<<endl;
    	for(int i=0;i<ve.size();i++){
    		ans *=ve[i];
    		ans %=mod;
    	}
    	ans *= rui[w];
    	ans %= mod;
    	  	
    	cout<<ans<<endl;
    	
  //  	printf("%.10f\n",ans[n-1]);
    	return 0;
    }

Measured memory_bytes: 2428