#include <cstdio>
#define repi(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,a) repi(i,0,a)
#define all(a) (a).begin(), (a).end()

constexpr int MAX_N = 2000, mod = 1000000007;

int N;
int e[MAX_N];
int dp[MAX_N+10][2][2][2][2];

int main()
{
  scanf( "%d", &N );
  rep( i, N )
    scanf( "%d", e+i );

  if( !e[0] )
    dp[0][0][0][0][0] = dp[0][0][0][1][0] = dp[0][1][1][1][1] = 1;
  else
    dp[0][0][1][0][0] = dp[0][0][1][1][0] = dp[0][1][0][0][1] = dp[0][1][0][1][1] = dp[0][1][1][0][1] = 1;

  rep( i, N-1 ) rep( k, 2 ) rep( l, 2 )
  {
    if( !e[i+1] )
    {
      dp[i+1][0][0][k][l] += dp[i][0][0][k][l];
      dp[i+1][0][0][k][l] %= mod;
      dp[i+1][0][0][k][l] += dp[i][1][0][k][l];
      dp[i+1][0][0][k][l] %= mod;
      dp[i+1][1][1][k][l] += dp[i][1][1][k][l];
      dp[i+1][1][1][k][l] %= mod;
    }
    else
    {
      dp[i+1][0][1][k][l] += dp[i][0][0][k][l];
      dp[i+1][0][1][k][l] %= mod;
      dp[i+1][0][1][k][l] += dp[i][1][0][k][l];
      dp[i+1][0][1][k][l] %= mod;
      dp[i+1][1][0][k][l] += dp[i][0][1][k][l];
      dp[i+1][1][0][k][l] %= mod;
      dp[i+1][1][1][k][l] += dp[i][0][1][k][l];
      dp[i+1][1][1][k][l] %= mod;
      dp[i+1][1][0][k][l] += dp[i][1][1][k][l];
      dp[i+1][1][0][k][l] %= mod;
    }
  }

  int ans = 0;

  rep( j, 2 ) rep( k, 2 )
    ans = (ans + dp[N-1][k][j][k][j]) % mod;

  printf( "%d\n", ans );

  return 0;
}