I just made a mistake while participating in a coding contest in Vietnam and I came across a simple dp problem, the problem was: https://lqdoj.edu.vn/problem/dutpc2a : test of the contest organizers that I participated in, https://atcoder.jp/contests/dp/tasks/dp_h: The problem I encountered is the same as the one on atcoder

But I accepted this problem on atcoder but when I submitted the test from the contest organizer that I joined, I was wrong. And the problem I have is when I compare with the character '.' I got wrong but compare with '#' then accepted. Can someone explain to me why when I compare the character '.' wrong?

I think there are a few caveats when compared to the '.' and some other special characters, if my thinking is correct, someone can explain it to me.

this my code wrong:

char v[nax][nax]; int dp[nax][nax];

int32_t main() {

FASTIO

int n, m;

cin >> n >> m;

for (int i = 1; i <= n; ++i) {
  for (int j = 1; j <= m; ++j) {
    cin >> v[i][j];
  }
}
for (int i = 1; i <= n; ++i) {
  for (int j = 1; j <= m; ++j) {
    if (i == 1 && j == 1)
    {
      dp[i][j]=1;
    }
    else
      if (v[i][j] == '.')
      {
        dp[i][j] = (dp[i - 1][j] + dp[i][j - 1]) % mod;
      }
  }
}
cout << dp[n][m];
return 0;

}

and my code accepted:

char v[nax][nax]; int dp[nax][nax];

int32_t main() {

FASTIO

int n, m;

cin >> n >> m;

for (int i = 1; i <= n; ++i) {
  for (int j = 1; j <= m; ++j) {
    cin >> v[i][j];
  }
}
for (int i = 1; i <= n; ++i) {
  for (int j = 1; j <= m; ++j) {
    if (i == 1 && j == 1)
    {
      dp[i][j]=1;
    }
    else
    {
      if (v[i][j] != '#')
      {
        dp[i][j] = (dp[i - 1][j] + dp[i][j - 1]) % mod;
      }
    }
  }
}
cout << dp[n][m];
return 0;

}

Thanks for the help and sorry for my english!