Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

sys.setrecursionlimit(10 ** 7)


MOD = 1000000007  # type: int


def solve(N: int, M: int, a: "List[int]"):
    ways_to_stairs = {}
    stairs = []
    for i in range(N + 1):
        stairs.append(True)
    for i in a:
        stairs[i] = False

    routes_to_stairs = {0: 1}
    if 1 not in a:
        routes_to_stairs[1] = 1

    for i in range(N + 1):
        if i <= 1:
            continue
        if stairs[i]:
            ways = 0
            if stairs[i - 1]:
                ways += routes_to_stairs[i - 1]
            if stairs[i - 2]:
                ways += routes_to_stairs[i - 2]
        routes_to_stairs[i] = ways

    print(routes_to_stairs[N] % MOD)
    return


# Generated by 1.1.5 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    a = [int(next(tokens)) for _ in range(M)]  # type: "List[int]"
    solve(N, M, a)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

sys.setrecursionlimit(10 ** 7)


MOD = 1000000007  # type: int


def solve(N: int, M: int, a: "List[int]"):
    ways_to_stairs = {}
    stairs = []
    for i in range(N + 1):
        stairs.append(True)
    for i in a:
        stairs[i] = False

    routes_to_stairs = {0: 1}
    if 1 not in a:
        routes_to_stairs[1] = 1

    for i in range(N + 1):
        if i <= 1:
            continue
        ways = 0
        if stairs[i]:
            if stairs[i - 1]:
                ways += routes_to_stairs[i - 1]
            if stairs[i - 2]:
                ways += routes_to_stairs[i - 2]
        routes_to_stairs[i] = ways

    print(routes_to_stairs[N] % MOD)
    return


# Generated by 1.1.5 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    a = [int(next(tokens)) for _ in range(M)]  # type: "List[int]"
    solve(N, M, a)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

MOD = 1000000007  # type: int


def solve(N: int, M: int, a: "List[int]"):
    ways_to_stairs = {}
    stairs = []
    for i in range(N + 1):
        stairs.append(True)
    for i in a:
        stairs[i] = False

    routes_to_stairs = {0: 1}
    if 1 not in a:
        routes_to_stairs[1] = 1

    for i in range(N + 1):
        if i <= 1:
            continue
        if stairs[i]:
            ways = 0
            if stairs[i - 1]:
                ways += routes_to_stairs[i - 1]
            if stairs[i - 2]:
                ways += routes_to_stairs[i - 2]
        routes_to_stairs[i] = ways

    print(routes_to_stairs[N] % MOD)
    return


# Generated by 1.1.5 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    a = [int(next(tokens)) for _ in range(M)]  # type: "List[int]"
    solve(N, M, a)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

sys.setrecursionlimit(10 ** 7)


MOD = 1000000007  # type: int


def solve(N: int, M: int, a: "List[int]"):
    ways_to_stairs = {}
    stairs = []
    for i in range(N + 1):
        stairs.append(True)
    for i in a:
        stairs[i] = False

    routes_to_stairs = {0: 1}
    if 1 not in a:
        routes_to_stairs[1] = 1

    for i in range(N + 1):
        if i <= 1:
            continue
        ways = 0
        if stairs[i]:
            if stairs[i - 1]:
                ways += routes_to_stairs[i - 1]
            if stairs[i - 2]:
                ways += routes_to_stairs[i - 2]
        routes_to_stairs[i] = ways

    print(routes_to_stairs[N] % MOD)
    return


# Generated by 1.1.5 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    a = [int(next(tokens)) for _ in range(M)]  # type: "List[int]"
    solve(N, M, a)

if __name__ == '__main__':
    main()