Buggy python submission (verdict: Runtime Error):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)
 
out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]
 
for s,t in ST:
    out_edge[s].append(t)
    in_edge[t].append(s)
 
out_deg = [len(x) for x in out_edge]
 
arrive_prob = [0]*(N+1)
arrive_prob[1] = 1.0
for n in range(2,N+1):
    arrive_prob[n] = sum(arrive_prob[p]/out_deg[p] for p in in_edge[n])
 
# その頂点に辿り着くまでの回数期待値
# いらなかったっぽい
# expected_to = [0]*(N+1)
# for n in range(2,N+1):
#     expected_to[n] = 1 + sum(expected_to[p]*arrive_prob[p]/out_deg[p] for p in in_edge[n])/arrive_prob[n]
 
# その頂点に辿り着いてからの回数期待値
expected_from = [0]*(N+1)
for n in range(N-1,0,-1):
    expected_from[n] = 1+sum(expected_from[c] for c in out_edge[n])/out_deg[n]
 
current_time = expected_from[1]
 
improve = 0
for n in range(1,N):
    if out_deg[n] == 1:
        continue
    x = expected_from[n]
    d = out_deg[n]
    y = (d*(x-1)-max(expected_from[c] for c in out_edge[n]))/(d-1) + 1
    value = (x-y)*arrive_prob[n]
    if improve < value:
        improve = value
 
answer = current_time - improve
print(answer)
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)

out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]

for s,t in ST:

Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)

out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]

for s,t in ST:
    out_edge[s].append(t)
    in_edge[t].append(s)

out_deg = [len(x) for x in out_edge]

arrive_prob = [0]*(N+1)
arrive_prob[1] = 1.0
for n in range(2,N+1):
    arrive_prob[n] = sum(arrive_prob[p]/out_deg[p] for p in in_edge[n])

# その頂点に辿り着くまでの回数期待値
# いらなかったっぽい
# expected_to = [0]*(N+1)
# for n in range(2,N+1):
#     expected_to[n] = 1 + sum(expected_to[p]*arrive_prob[p]/out_deg[p] for p in in_edge[n])/arrive_prob[n]

# その頂点に辿り着いてからの回数期待値
expected_from = [0]*(N+1)
for n in range(N-1,0,-1):
    expected_from[n] = 1+sum(expected_from[c] for c in out_edge[n])/out_deg[n]

current_time = expected_from[1]

improve = 0
for n in range(1,N):
    if out_deg[n] == 1:
        continue
    x = expected_from[n]
    d = out_deg[n]
    y = (d*(x-1)-max(expected_from[c] for c in out_edge[n]))/(d-1) + 1
    value = (x-y)*arrive_prob[n]
    if improve < value:
        improve = value

answer = current_time - improve
print(answer)

Buggy python submission (verdict: Runtime Error):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)
 
out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]
 
for s,t in ST:
    out_edge[s].append(t)
    in_edge[t].append(s)

out_deg = [len(x) for x in out_edge]
 
arrive_prob = [0]*(N+1)
arrive_prob[1] = 1.0
for n in range(2,N+1):
    arrive_prob[n] = sum(arrive_prob[p]/out_deg[p] for p in in_edge[n])

expected_from = [0]*(N+1)
for n in range(N-1,0,-1):
    expected_from[n] = 1+sum(expected_from[c] for c in out_edge[n])/out_deg[n]

current_time = expected_from[1]
 
improve = 0
for n in range(1,N):
    if out_deg[n] == 1:
        continue
    x = expected_from[n]
    d = out_deg[n]
    y = (d*(x-1)-max(expected_from[c] for c in out_edge[n]))/(d-1) + 1
    value = (x-y)*arrive_prob[n]
    if improve < value:
        improve = value

answer = current_time - improve
print(answer)
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)

out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]

for s,t in ST:

Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N,M = map(int,readline().split())
m = map(int,read().split())
ST = zip(m,m)

out_edge = [[] for _ in range(N+1)]
in_edge = [[] for _ in range(N+1)]

for s,t in ST:
    out_edge[s].append(t)
    in_edge[t].append(s)

out_deg = [len(x) for x in out_edge]

arrive_prob = [0]*(N+1)
arrive_prob[1] = 1.0
for n in range(2,N+1):
    arrive_prob[n] = sum(arrive_prob[p]/out_deg[p] for p in in_edge[n])

# その頂点に辿り着くまでの回数期待値
# いらなかったっぽい
# expected_to = [0]*(N+1)
# for n in range(2,N+1):
#     expected_to[n] = 1 + sum(expected_to[p]*arrive_prob[p]/out_deg[p] for p in in_edge[n])/arrive_prob[n]

# その頂点に辿り着いてからの回数期待値
expected_from = [0]*(N+1)
for n in range(N-1,0,-1):
    expected_from[n] = 1+sum(expected_from[c] for c in out_edge[n])/out_deg[n]

current_time = expected_from[1]

improve = 0
for n in range(1,N):
    if out_deg[n] == 1:
        continue
    x = expected_from[n]
    d = out_deg[n]
    y = (d*(x-1)-max(expected_from[c] for c in out_edge[n]))/(d-1) + 1
    value = (x-y)*arrive_prob[n]
    if improve < value:
        improve = value

answer = current_time - improve
print(answer)