Buggy python submission (verdict: Runtime Error):
#!/usr/bin python3
# -*- coding: utf-8 -*-

mod = 10**9+7               # mod素数
N = 1000002                   # 出力の制限
g1 = [1]*(N+1)              # 元テーブル
g2 = [1]*(N+1)              # 逆元テーブル
for i in range( 2, N + 1 ): # 準備
    g1[i] = ( g1[i-1] * i ) % mod
    g2[i] = ( g2[i-1] * pow(i,mod-2,mod) ) % mod

def nCr(n, r, mod):
    if ( r<0 or r>n ):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % mod

def main():
#    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCr(i+Nk-1,i,mod) * pw25 * pw26
        ret %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    for i in range(K+1):
        ret += nCk * pow(25,i,mod) * pow(26,K-i,mod)
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
    print(ret)

if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin python3
# -*- coding: utf-8 -*-

mod = 10**9+7
N = 10**6                   # 出力の制限
g1 = [1]*(N+1)              # 元テーブル
g2 = [1]*(N+1)              # 逆元テーブル
for i in range( 2, N + 1 ): # 準備
    g1[i] = ( g1[i-1] * i ) % mod
    g2[i] = ( g2[i-1] * pow(i,mod-2,mod) ) % mod

def nCr(n, r, mod):
    if ( r<0 or r>n ):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % mod

def main():
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    for i in range(K+1):
        ret += nCr(i+Nk-1, Nk-1, mod) * pow(25,i,mod) * pow(26,K-i,mod)
        ret %= mod
    print(ret)

if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin python3
# -*- coding: utf-8 -*-

mod = 10**9+7
N = 10**5                   # 出力の制限
g1 = [1]*(N+1)              # 元テーブル
g2 = [1]*(N+1)              # 逆元テーブル
for i in range( 2, N + 1 ): # 準備
    g1[i] = ( g1[i-1] * i ) % mod
    g2[i] = ( g2[i-1] * pow(i,mod-2,mod) ) % mod

def nCr(n, r, mod):
    if ( r<0 or r>n ):
        return 0
    r = min(r, n-r)
    return g1[n] * g2[r] * g2[n-r] % mod

def main():
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    for i in range(K+1):
        ret += nCr(i+Nk-1, Nk-1, mod) * pow(25,i,mod) * pow(26,K-i,mod)
        ret %= mod
    print(ret)

if __name__ == '__main__':
    main()

Fixed submission:
#!/usr/bin python3
# -*- coding: utf-8 -*-

def main():
    mod = 10**9+7
    K = int(input())
    S = input()
    Nk = len(S)
    ret = 0
    nCk = 1
    pw25 = 1
    pw26 = pow(26,K,mod)
    inv26 = pow(26,mod-2,mod)
    for i in range(K+1):
        ret += nCk * pw25 * pw26
        ret %= mod
        nCk *= (i+Nk) * pow(i+1,mod-2,mod)
        nCk %= mod
        pw25 = (pw25*25)%mod
        pw26 = (pw26*inv26)%mod

    print(ret)

if __name__ == '__main__':
    main()