Problem p03013 (python)

Fixed submission:
import sys

N, M =  map(int, input().split())
a_list = [int(input()) for _ in range(M)]

if N == 1:
    print(1)

else:
    a = 1
    ans = 1

    if M == 0:
        b = 1
        for i in range(2, N+1):
            ans = (a+b)%1000000007
            a, b = b, ans

    elif M == 1:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if a_list[0] == i:
                a, b = b, 0
            else:
                ans = (a+b)%1000000007
                a, b = b, ans

    else:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if m < M-1:
                if a_list[m+1] - a_list[m] == 1:
                    print(0)
                    sys.exit()

                if a_list[m] == i:
                    a, b = b, 0
                    m += 1
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

            else:
                if a_list[m] == i:
                    a, b = b, 0
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

    print(ans)

Buggy submission (verdict: Runtime Error):
import sys

N, M =  map(int, input().split())
a_list = [int(input()) for _ in range(M)]

if N == 1:
    print(1)

else:
    a = 1
    ans = 1

    if M == 0:
        ans = (a+b)%1000000007
        a, b = b, ans

    elif M == 1:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if a_list[0] == i:
                a, b = b, 0
            else:
                ans = (a+b)%1000000007
                a, b = b, ans

    else:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if m < M-1:
                if a_list[m+1] - a_list[m] == 1:
                    print(0)
                    sys.exit()

                if a_list[m] == i:
                    a, b = b, 0
                    m += 1
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

            else:
                if a_list[m] == i:
                    a, b = b, 0
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

    print(ans)

Buggy submission (verdict: Runtime Error):
import sys

N, M =  map(int, input().split())
a_list = [int(input()) for _ in range(M)]

if N == 1:
    print(1)

else:
    a = 1
    ans = 1

    if M == 0:
        ans = (a+b)%1000000007
        a, b = b, ans

    elif M == 1:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if a_list[0] == i:
                a, b = b, 0
            else:
                ans = (a+b)%1000000007
                a, b = b, ans

    else:
        if a_list[0] == 1:
            b = 0
            m = 1
        else:
            b = 1
            m = 0
        for i in range(2, N+1):
            if m < M-1:
                if a_list[m+1] - a_list[m] == 1:
                    print(0)
                    sys.exit()

                if a_list[m] == i:
                    a, b = b, 0
                    m += 1
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

            else:
                if a_list[m] == i:
                    a, b = b, 0
                else:
                    ans = (a+b)%1000000007
                    a, b = b, ans

print(ans)