Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
h,w = list(map(int,input().split()))
s = []
for i in range(h):
    sh = list(input())
    s.append(sh)   
ki = [[] for i in range(h*w)]
def ichi(a,b):
    return a + b*h
for a in range(h):
    for b in range(w):
        if s[a][b] == ".":
            if  a+1 < h and s[a+1][b] == "." :
                ki[ichi(a+1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a+1,b))
            if  a-1 >= 0 and s[a-1][b] == ".":
                ki[ichi(a-1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a-1,b))
            if b-1 >= 0 and s[a][b-1] == ".":
                ki[ichi(a,b-1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b-1))
            if b+1 < w and s[a][b+1] == "." :
                ki[ichi(a,b+1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b+1))
        else:
            pass
for i in range(w*h):
    ki[i] = list(set(ki[i]))
ans = [0]
for i in range(w*h):
    ikeru = deque([i])
    sumi = []
    kaisuu = [-1]*(w*h)
    kaisuu[i] = 0
    while ikeru:
        j = ikeru.popleft()
        sumi.append(j)
        for k in range(len(ki[j])):
            if not ki[j][k] in sumi:
                ikeru.append(ki[j][k])
                if kaisuu[ki[j][k]] > kaisuu[j] + 1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1
                elif kaisuu[ki[j][k]] == -1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1 
#        print(i,kaisuu,ikeru)
    ans.append(max(kaisuu))
#print(ans)
print(max(ans))        

Fixed submission:
from collections import deque
h,w = list(map(int,input().split()))
s = []
for i in range(h):
    sh = list(input())
    s.append(sh)
ki = [[] for i in range(h*w)]
def ichi(a,b):
    return a + b*h
for a in range(h):
    for b in range(w):
        if s[a][b] == ".":
            if  a+1 < h and s[a+1][b] == "." :
                ki[ichi(a+1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a+1,b))
            if  a-1 >= 0 and s[a-1][b] == ".":
                ki[ichi(a-1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a-1,b))
            if b-1 >= 0 and s[a][b-1] == ".":
                ki[ichi(a,b-1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b-1))
            if b+1 < w and s[a][b+1] == "." :
                ki[ichi(a,b+1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b+1))
        else:
            pass
for i in range(w*h):
    ki[i] = list(set(ki[i]))
ans = [0]
for i in range(w*h):
    ikeru = deque([i])
    sumi = []
    kaisuu = [-1]*(w*h)
    kaisuu[i] = 0
    while ikeru:
        j = ikeru.popleft()
        sumi.append(j)
        for k in range(len(ki[j])):
            if ki[j][k] in sumi:
                pass
            else:
                if ki[j][k] not in ikeru:
                    ikeru.append(ki[j][k])
                if kaisuu[ki[j][k]] > kaisuu[j] + 1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1
                elif kaisuu[ki[j][k]] == -1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1 
#        print(i,kaisuu,ikeru)
    ans.append(max(kaisuu))
#print(ans)
print(max(ans))

Buggy python submission (verdict: Time Limit Exceeded):
#n = int(input())
h,w = list(map(int,input().split()))
s = []
for i in range(h):
    sh = list(input())
    s.append(sh)
#ki = ["n"]*(h*w)    
ki = [[] for i in range(h*w)]
def ichi(a,b):
    return a + b*h
for a in range(h):
    for b in range(w):
        if s[a][b] == ".":
            if  a+1 < h and s[a+1][b] == "." :
                ki[ichi(a+1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a+1,b))
            if  a-1 >= 0 and s[a-1][b] == ".":
                ki[ichi(a-1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a-1,b))
            if b-1 >= 0 and s[a][b-1] == ".":
                ki[ichi(a,b-1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b-1))
            if b+1 < w and s[a][b+1] == "." :
                ki[ichi(a,b+1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b+1))
        else:
            pass
for i in range(w*h):
    ki[i] = list(set(ki[i]))
ans = [0]
for i in range(w*h):
    ikeru = [i]
    sumi = []
    kaisuu = [-1]*(w*h)
    kaisuu[i] = 0
    while len(ikeru) > 0:
        j = ikeru.pop(0)
        sumi.append(j)
        for k in range(len(ki[j])):
            if not ki[j][k] in sumi:
                ikeru.append(ki[j][k])
                if kaisuu[ki[j][k]] > kaisuu[j] + 1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1
                elif kaisuu[ki[j][k]] == -1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1 
#        print(i,kaisuu,ikeru)
    ans.append(max(kaisuu))
#print(ans)
print(max(ans))        

Fixed submission:
from collections import deque
h,w = list(map(int,input().split()))
s = []
for i in range(h):
    sh = list(input())
    s.append(sh)
ki = [[] for i in range(h*w)]
def ichi(a,b):
    return a + b*h
for a in range(h):
    for b in range(w):
        if s[a][b] == ".":
            if  a+1 < h and s[a+1][b] == "." :
                ki[ichi(a+1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a+1,b))
            if  a-1 >= 0 and s[a-1][b] == ".":
                ki[ichi(a-1,b)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a-1,b))
            if b-1 >= 0 and s[a][b-1] == ".":
                ki[ichi(a,b-1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b-1))
            if b+1 < w and s[a][b+1] == "." :
                ki[ichi(a,b+1)].append(ichi(a,b))
                ki[ichi(a,b)].append(ichi(a,b+1))
        else:
            pass
for i in range(w*h):
    ki[i] = list(set(ki[i]))
ans = [0]
for i in range(w*h):
    ikeru = deque([i])
    sumi = []
    kaisuu = [-1]*(w*h)
    kaisuu[i] = 0
    while ikeru:
        j = ikeru.popleft()
        sumi.append(j)
        for k in range(len(ki[j])):
            if ki[j][k] in sumi:
                pass
            else:
                if ki[j][k] not in ikeru:
                    ikeru.append(ki[j][k])
                if kaisuu[ki[j][k]] > kaisuu[j] + 1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1
                elif kaisuu[ki[j][k]] == -1:
                    kaisuu[ki[j][k]] = kaisuu[j] + 1 
#        print(i,kaisuu,ikeru)
    ans.append(max(kaisuu))
#print(ans)
print(max(ans))