#include <set>
#include <map>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <string>
#include <vector>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>using namespace std;typedef long long ll;
typedef unsigned long long ull;#define debug puts("here")
#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define REP(i,a,b) for(int i=a;i<=b;i++)
#define foreach(i,vec) for(unsigned i=0;i<vec.size();i++)
#define pb push_back
#define RD(n) scanf("%d",&n)
#define RD2(x,y) scanf("%d%d",&x,&y)
#define RD3(x,y,z) scanf("%d%d%d",&x,&y,&z)
#define RD4(x,y,z,w) scanf("%d%d%d%d",&x,&y,&z,&w)
#define All(vec) vec.begin(),vec.end()
#define MP make_pair
#define PII pair<int,int>
#define PQ priority_queue
#define cmax(x,y) x = max(x,y)
#define cmin(x,y) x = min(x,y)
#define Clear(x) memset(x,0,sizeof(x))
/*#pragma comment(linker, "/STACK:1024000000,1024000000")int size = 256 << 20; // 256MB
char *p = (char*)malloc(size) + size;
__asm__("movl %0, %%esp\n" :: "r"(p) );*//******** program ********************/const int MAXN = 100005;int a[MAXN];
int dp[MAXN];
int q[MAXN];
int n;void cc(){int top = 0;rep1(i,n){if(top==0||dp[top]<a[i]){dp[++top] = a[i];continue;}int l = 1 , r = top;int ans = 1;while(l<=r){int mid = (l+r)>>1;if(dp[mid]<a[i])l = mid+1;else{ans = mid;r = mid-1;}}dp[ans] = a[i];}cout<<top<<endl;
}int main(){#ifndef ONLINE_JUDGEfreopen("sum.in","r",stdin);//freopen("sum.out","w",stdout);
#endifwhile(cin>>n){rep1(i,n)RD(a[i]);cc();}return 0;
}

#include <set>
#include <map>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <string>
#include <vector>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>using namespace std;typedef long long ll;
typedef unsigned long long ull;#define debug puts("here")
#define rep(i,n) for(int i=0;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define REP(i,a,b) for(int i=a;i<=b;i++)
#define foreach(i,vec) for(unsigned i=0;i<vec.size();i++)
#define pb push_back
#define RD(n) scanf("%d",&n)
#define RD2(x,y) scanf("%d%d",&x,&y)
#define RD3(x,y,z) scanf("%d%d%d",&x,&y,&z)
#define RD4(x,y,z,w) scanf("%d%d%d%d",&x,&y,&z,&w)
#define All(vec) vec.begin(),vec.end()
#define MP make_pair
#define PII pair<int,int>
#define PQ priority_queue
#define cmax(x,y) x = max(x,y)
#define cmin(x,y) x = min(x,y)
#define Clear(x) memset(x,0,sizeof(x))
/*#pragma comment(linker, "/STACK:1024000000,1024000000")int size = 256 << 20; // 256MB
char *p = (char*)malloc(size) + size;
__asm__("movl %0, %%esp\n" :: "r"(p) );*//******** program ********************/const int MAXN = 2005;
const int MOD = 1e9+7;ll dp[MAXN][MAXN];
int a[MAXN],n;
bool use[MAXN];
ll p[MAXN];ll dfs(int x,int y,int res){if(res<=0) return p[y];if(x==1)return p[y]*y%MOD;if(x==0)return p[y];if(~dp[x][y])return dp[x][y];ll tmp = (x-1);tmp *= dfs(x-2,y+1,res-2);tmp += dfs(x-1,y,res-1)*y;return dp[x][y] = tmp%MOD;
}int main(){#ifndef ONLINE_JUDGEfreopen("sum.in","r",stdin);//freopen("sum.out","w",stdout);
#endifp[0] = p[1] = 1;for(int i=2;i<MAXN;i++)p[i] = p[i-1]*i%MOD;while(cin>>n){Clear(use);rep1(i,n){RD(a[i]);if(~a[i])use[ a[i] ] = true;}int x = 0 , y = 0;rep1(i,n){if(a[i]==-1){if(use[i])y ++;else    x ++;}}memset(dp,-1,sizeof(dp));cout<<dfs(x,y,x+y)<<endl;}return 0;
}