#include <bits/stdc++.h>
#define FOR(i,a,b) for( ll i = (a); i < (ll)(b); i++ )
#define REP(i,n) FOR(i,0,n)
#define YYS(x,arr) for(auto& x:arr)
#define ALL(x) (x).begin(),(x).end()
#define SORT(x) sort( (x).begin(),(x).end() )
#define REVERSE(x) reverse( (x).begin(),(x).end() )
#define UNIQUE(x) (x).erase( unique( ALL( (x) ) ) , (x).end() )
#define PW(x) (1LL<<(x))
#define SZ(x) ((ll)(x).size())
#define SHOW(x) cout << #x << " = " << x << endl
#define SHOWA(x,n) for( int yui = 0; yui < n; yui++ ){ cout << x[yui] << " "; } cout << endl
#define pb emplace_back
#define fi first
#define se second
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef pair<int,int> pi;
typedef pair<ll,ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<bool> vb;
typedef vector<ld> vd;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<vpl> gr;
typedef vector<vl> ml;
typedef vector<vd> md;
typedef vector<vi> mi;
const ll INF = (ll)1e9 + 10;
const ll INFLL = (ll)1e18 + 10;
const ld EPS = 1e-12;
const ll MOD = 1e9+7;
template<class T> T &chmin( T &a , const T &b ){ return a = min(a,b); }
template<class T> T &chmax( T &a , const T &b ){ return a = max(a,b); }
template<class T> inline T sq( T a ){ return a * a; }
ll in(){ long long int x; scanf( "%lld" , &x ); return x; }
char yuyushiki[1000010]; string stin(){ scanf( "%s" , yuyushiki ); return yuyushiki; }
// head
struct Mod{
unsigned n;
Mod() : n(0){}
Mod( ll x ){
if( x < 0 ) n = x%MOD+MOD;
else n = x%MOD;
}
};
Mod operator + ( Mod a , Mod b ){ return Mod( a.n + b.n ); }
Mod operator +=( Mod &a , Mod b ){ return a = a + b; }
Mod operator - ( Mod a ){ return Mod( MOD - a.n ); }
Mod operator - ( Mod a , Mod b ){ return Mod( a.n + MOD - b.n ); }
Mod operator -=( Mod &a , Mod b ){ return a = a - b; }
Mod operator * ( Mod a , Mod b ){ return Mod( (ll)a.n * b.n ); }
Mod operator *=( Mod &a , Mod b ){ return a = a * b; }
Mod modpow( Mod x , ll k ){
Mod res = 1;
while( k ){
if( k & 1 ) res *= x;
k /= 2;
x *= x;
}
return res;
}
ll extgcd( ll a , ll b , ll &x , ll &y ){
ll d = a;
if( b != 0 ){
d = extgcd( b , a % b , y , x );
y -= a / b * x;
} else {
x = 1, y = 0;
}
return d;
}
Mod inv( Mod a ){ ll x, y; assert( extgcd( a.n , MOD , x , y ) == 1 ); return Mod( x ); }
Mod operator / ( Mod a , Mod b ){ return Mod( (ll)a.n * inv(b).n ); }
Mod operator /=( Mod &a , Mod b ){ return a = a / b; }
int n;
int a[100010];
int main(){
n = in();
REP( i , n ){
a[i] = in();
}
int s = -1;
int pr = -1;
Mod ans = 1;
REP( i , n ){
if( pr == a[i] - 1 ){
ans *= ( i - s );
s++;
} else {
pr = pr + 2;
}
}
while( s < n-1 ){
ans *= n - 1 - s;
s++;
}
cout << ans.n << endl;
return 0;
}