#include<bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define REP(i, n) FOR(i, 0, n)
#define _ << " " <<
#define sz(x) ((int) x.size())
#define pb(x) push_back(x)
#define TRACE(x) cerr << #x << " = " << x << endl

typedef long long ll;
typedef pair<int, int> point;

const int mod = 1e9 + 7;

int add(int x, int y) {x += y; if(x >= mod) return x - mod; return x;}
int sub(int x, int y) {x -= y; if(x < 0) return x + mod; return x;}
int mul(int x, int y) {return (ll) x * y % mod;}

const int MAXN = 505;

int n, a[MAXN], dp[MAXN][MAXN];

int main(){
  ios_base::sync_with_stdio(false); cin.tie(0);

  cin >> n;
  REP(i, n)
    cin >> a[i];

  if(a[0] != 0){
    cout << 0;
    return 0;
  }

  dp[0][0] = 1;
  FOR(i, 1, n){
    int mx = min(i, n - 1 - i);
    FOR(j, 0, n){
      if(j > mx) continue;
      if(a[i] != -1 && j != a[i]) continue;

      if(j > 0)
        dp[i][j] = add(dp[i][j], dp[i - 1][j - 1]);
      dp[i][j] = add(dp[i][j], dp[i - 1][j]);
      dp[i][j] = add(dp[i][j], dp[i - 1][j + 1]);
    }
  }
  cout << dp[n - 1][0];
}