//#include <bits/stdc++.h>
//using namespace std;
//typedef long long ll;
//inline ll read()
//{
//    ll s=0;
//    bool f=0;
//    char ch=' ';
//    while(!isdigit(ch))
//    {
//        f|=(ch=='-'); ch=getchar();
//    }
//    while(isdigit(ch))
//    {
//        s=(s<<3)+(s<<1)+(ch^48); ch=getchar();
//    }
//    return (f)?(-s):(s);
//}
//#define R(x) x=read()
//inline void write(ll x)
//{
//    if(x<0)
//    {
//        putchar('-'); x=-x;
//    }
//    if(x<10)
//    {
//        putchar(x+'0'); return;
//    }
//    write(x/10);
//    putchar((x%10)+'0');
//    return;
//}
//#define W(x) write(x),putchar(' ')
//#define Wl(x) write(x),putchar('\n')
//const int N=3005;
//const ll Mod=1000000007;
//int n,a[N];
//ll dp[N][N],Sum[N];
//inline void Ad(ll &x,ll y) {x=x+y; x-=(x>=Mod)?Mod:0;}
//int main()
//{
//    freopen("dfs.in","r",stdin);
//    freopen("dfs.out","w",stdout);
//    int i,j;
//    R(n);
//    for(i=1;i<=n;i++) R(a[i]);
//    if((a[1]!=-1)&&(a[1]!=0)) return puts("0"),0;
//    dp[1][0]=1; Sum[n+1]=0; for(i=n;i>=0;i--) Ad(Sum[i],Sum[i+1]+dp[1][i]);
//    for(i=2;i<=n;i++)
//    {
//        if(a[i]!=-1) Ad(dp[i][a[i]],Sum[a[i]-1]);
//        else for(j=1;j<=n;j++) Ad(dp[i][j],Sum[j-1]);
//        for(j=n;j>=0;j--) {Sum[j]=Sum[j+1]+dp[i][j]; Sum[j]-=((Sum[j]>=Mod)?Mod:0);}
//    }
//    ll ans=0;
//    for(i=0;i<=n;i++) Ad(ans,dp[n][i]);
//    Wl(ans);
//    return 0;
//}
///*
//input
//2
//1 -1
//output
//0
//
//input
//4
//0 -1 1 -1
//output
//2
//
//input
//5
//-1 -1 -1 -1 -1
//output
//14
//*/

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
inline ll read()
{ll s=0;bool f=0;char ch=' ';while(!isdigit(ch)){f|=(ch=='-'); ch=getchar();}while(isdigit(ch)){s=(s<<3)+(s<<1)+(ch^48); ch=getchar();}return (f)?(-s):(s);
}
#define R(x) x=read()
inline void write(ll x)
{if(x<0){putchar('-'); x=-x;}if(x<10){putchar(x+'0'); return;}write(x/10);putchar((x%10)+'0');return;
}
#define W(x) write(x),putchar(' ')
#define Wl(x) write(x),putchar('\n')
const ll Mod=1000000007;
const int N=4000005;
int n,a[N];
ll Jiec[N],Invj[N];
inline ll Ksm(ll x,ll y)
{ll ans=1;while(y){if(y&1) ans=ans*x%Mod;x=x*x%Mod;y>>=1;}return ans;
}
inline ll C(ll n,ll m)
{if(n<m||m<0) return 0;return Jiec[n]*Invj[m]%Mod*Invj[n-m]%Mod;
}
int main()
{freopen("dfs.in","r",stdin);freopen("dfs.out","w",stdout);ll i,Last,ans;R(n);for(i=1;i<=n;i++) R(a[i]);Jiec[0]=Invj[0]=1ll;for(i=1;i<=2*n;i++){Jiec[i]=Jiec[i-1]*i%Mod; Invj[i]=Ksm(Jiec[i],Mod-2);}if((a[1]!=0)&&(a[1]!=-1)) return puts("0"),0;a[1]=0; Last=ans=a[n+1]=1;for(i=2;i<=n+1;i++) if(a[i]!=-1){if(a[i]==0) return puts("0"),0;int s=i-Last-1,x=i-Last-1-a[i]+1+a[Last];if(x<0) return puts("0"),0;ans=1ll*ans*(C(s+x,x)-C(s+x,x-a[Last]-1))%Mod;Last=i;}ans+=(ans<0)?Mod:0;Wl(ans);return 0;
}
/*
input
6
-1 -1 -1 -1 -1 -1
output
42
*/