Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h in d and d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1


Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
for i in range(1, 1000000):
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(i)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break


Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [set([y]), False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [set([y]), False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = [S]
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in list(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [set([y]), False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return list(history)

history = [S]
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in reversed(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1

Buggy python submission (verdict: Runtime Error):
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].append(y)
    else:
        d[x] = [[y], False]

def next_node(start):
    history = []
    for s in start:
        if s in d:
            history.extend(d[s][0])
    return list(set(history))

history = [S]
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in reversed(history):
        if d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1
    

Fixed submission:
N, M = [int(x) for x in input().split()]
links = [tuple(map(int, input().split())) for _ in range(M)]
S, T = [int(x) for x in input().split()]

d = {}
for x, y in links:
    if x in d:
        d[x][0].add(y)
    else:
        d[x] = [{y}, False]

def next_node(start):
    history = set()
    for s in start:
        if s in d:
            history |= d[s][0]
    return history

history = {S}
s = 1
while True:
    for _ in range(3):
        history = next_node(history)
    if T in history:
        print(s)
        break
    for h in tuple(history):
        if h not in d or d[h][1]:
            history.remove(h)
        else:
            d[h][1] = True
    if len(history) == 0:
        print(-1)
        break
    s += 1