求助，为什么过不了

https://ac.nowcoder.com/acm/problem/266841

n,m=map(int,input().split())
mp=[list(input()) for _ in range(n)]
vis=[[False]*1005 for _ in range(1005)]
l,r,u,b=m,0,0,n
cnt=0
ans=0

def dfs (x,y):
    global l,r,u,b,cnt
    l=min(l,y)
    r=max(r,y)
    u=max(u,x)
    b=min(b,x)
    cnt+=1
    vis[x][y]=True

    for i,j in [(x+1,y),(x,y+1),(x-1,y),(x,y-1)]:
        if 0<=i<n and 0<=j<m  and vis[i][j]!=True:
            dfs(i,j)
            
for i in range(n) :
    for j in range(m) :
        if mp[i][j]=="*":
            vis[i][j]=True

for i in range(n) :
    for j in range(m) :
        l,r,u,b=m,0,0,n
        cnt=0
        if vis[i][j]==0:
            dfs(i,j)
            if (r-l+1)*(u-b+1)==cnt : 
                ans+=1
print(ans)