Buggy python submission (verdict: Time Limit Exceeded):
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        if not able[u]:
            continue
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')


Fixed submission:
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        if not able[u]:
            continue
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for line in sys.stdin.readlines():
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')

Buggy python submission (verdict: Time Limit Exceeded):
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')


Fixed submission:
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        if not able[u]:
            continue
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for line in sys.stdin.readlines():
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')

Buggy python submission (verdict: Runtime Error):
import sys

sys.setrecursionlimit(200000)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        links[u][j].remove(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    la, lb = links[a], links[b]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)
    i, j = [(0, 1), (1, 0)][s[a] == 'B']
    if not lb[i] and lb[j]:
        able[b] = True
    lb[i].add(a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')


Fixed submission:
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        if not able[u]:
            continue
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for line in sys.stdin.readlines():
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')

Buggy python submission (verdict: Runtime Error):
def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        links[u][j].remove(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for _ in range(m):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    la, lb = links[a], links[b]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)
    i, j = [(0, 1), (1, 0)][s[a] == 'B']
    if not lb[i] and lb[j]:
        able[b] = True
    lb[i].add(a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')


Fixed submission:
import sys

sys.setrecursionlimit(200000)


def add_link(a, b):
    la = links[a]
    i, j = [(0, 1), (1, 0)][s[b] == 'B']
    if not la[i] and la[j]:
        able[a] = True
    la[i].add(b)


def close(v, i):
    j = int(s[v] == 'B')
    for u in links[v][i].copy():
        if not able[u]:
            continue
        links[u][j].discard(v)
        if not links[u][j]:
            able[u] = False
            close(u, j ^ 1)


n, m = map(int, input().split())
s = input()
links = [[set(), set()] for _ in range(n)]
able = [False] * n
for line in sys.stdin.readlines():
    a, b = map(int, line.split())
    a -= 1
    b -= 1
    add_link(a, b)
    add_link(b, a)

for v in range(n):
    if able[v]:
        continue
    close(v, bool(links[v][1]))

print('Yes' if any(able) else 'No')