Problem:
Score : 100 points

Problem Statement

There is a grid with H horizontal rows and W vertical columns.
Let (i, j) denote the square at the i-th row from the top and the j-th column from the left.

For each i and j (1 \leq i \leq H, 1 \leq j \leq W), Square (i, j) is described by a character a_{i, j}.
If a_{i, j} is ., Square (i, j) is an empty square; if a_{i, j} is #, Square (i, j) is a wall square.
It is guaranteed that Squares (1, 1) and (H, W) are empty squares.

Taro will start from Square (1, 1) and reach (H, W) by repeatedly moving right or down to an adjacent empty square.

Find the number of Taro's paths from Square (1, 1) to (H, W).
As the answer can be extremely large, find the count modulo 10^9 + 7.

Constraints

H and W are integers.

2 \leq H, W \leq 1000

a_{i, j} is . or #.

Squares (1, 1) and (H, W) are empty squares.

Input

Input is given from Standard Input in the following format:

H W
a_{1, 1}\ldotsa_{1, W}
:
a_{H, 1}\ldotsa_{H, W}

Output

Print the number of Taro's paths from Square (1, 1) to (H, W), modulo 10^9 + 7.

Sample Input 1

3 4
...#
.#..
....

Sample Output 1

3

There are three paths as follows:

Sample Input 2

5 2
..
#.
..
.#
..

Sample Output 2

0

There may be no paths.

Sample Input 3

5 5
..#..
.....
#...#
.....
..#..

Sample Output 3

24

Sample Input 4

20 20
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................
....................

Sample Output 4

345263555

Be sure to print the count modulo 10^9 + 7.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)


#include <iostream>
#include <vector>
#include <array>
#include <set>
#include <cmath>
#include <string>
#include <sstream>
#include <queue> 
#include <list>
#include <stack>
#include <unordered_map>
#include <algorithm> 
#include <numeric>  
#include <functional>
#include <cassert>
#include <bitset>
#include <cctype>  
#include <iomanip> 
#include <limits>  
#include <regex>
// std::numeric_limits<int>::max(), min()
// std::setprecision(X)
// std::islower(), std::isupper(), std::tolower, std::toupper
// std::accumulate(m.begin(), m.end(), 0LL);
// std::sort(begin(),end()) // 0, 1, 2, 3 ,4 (default, less);
// std::priority_queue<int, vector<int>, less<int>> que; // 4, 3, 2, 1 (defualt, less) : 
// if ( std::regex_match(S, std::regex("(dream|dreamer|erase|eraser)*")) ) std::cout << "YES" << std::endl;
 
#define rep(i, n) for(int i = 0; i < (int)(n);    i++)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
typedef long long ll;

ll gcd ( ll a, ll b ) { if ( b == 0 ) return a; else return gcd ( b, a%b ); }
ll lcm ( ll a, ll b ) { return a * b / gcd(a, b); }


// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const int mod = 1e9+7;
struct mint {
  typedef long long ll;
  ll x;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res+=a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res-=a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res*=a;
  }
  bool operator>(const mint a) const {
    mint res(*this);
    return res.x > a.x;
  }
  bool operator<(const mint a) const {
    mint res(*this);
    return res.x < a.x;
  }
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const {
    return pow(mod-2);
  }
  mint& operator/=(const mint a) {
    return (*this) *= a.inv();
  }
  mint operator/(const mint a) const {
    mint res(*this);
    return res/=a;
  }
};

std::vector<std::vector<char>> G;
std::vector<std::vector<mint>> dp;
std::vector<std::vector<bool>> used;

mint sol(int i, int j) {
  if ( i == 0 && j == 0 ) return mint(1);
  if ( i < 0  || j < 0  ) return mint(0);
  if ( used[i][j] ) return dp[i][j];
  mint k = 0;
  if ( i-1 >= 0 && G[i-1][j] == '.' ) k += sol(i-1,j);
  if ( j-1 >= 0 && G[i][j-1] == '.' ) k += sol(i,j-1);
  used[i][j] = true;
  return dp[i][j] = k;
}


int main (void) {
  std::cin.tie(0);  std::ios::sync_with_stdio(false);
  
  int H, W; std::cin >> H >> W;
  G.resize(H); rep(i,H) G[i].resize(W);
  rep(i,H) rep(j,W) std::cin >> G[i][j];

  dp.resize(H+1); rep(i,H+1) dp[i].resize(W+1, mint(0));
  used.resize(H+1); rep(i,H+1) used[i].resize(W+1);
  std::cout << sol(H-1,W-1).x << std::endl;
  return 0;
}