Buggy python submission (verdict: Runtime Error):
def main():
    N = int(input())

    i = 1
    sum = 0
    temp = 0
    while True:
        temp = 0
        i += 1
        sum += 1
        for j in range(1,i):
            temp += 26**j

        if N <= temp:
            break

    # 文字数はsum
    #for j in range(1,sum):
        #temp += 26**j
    #N -= temp

    ans = []
    flag = 0
    cnt = 0
    for j in range(sum):
        amari = N%26
        if amari == 0:
            flag = 1
            cnt += 1
            if cnt == 1:
                #print(amari,flag)
                ans.append(amari)
                N = N//26
                continue
        N = N//26
        #print(amari,flag)
        ans.append(amari-flag)

    #print(ans)

    ans2 = [0]*len(ans)
    dict = {1:'a',2:'b',3:'c',4:'d',5:'e',6:'f',7:'g',8:'h',9:'i',10:'j',11:'k',12:'l',13:'m',14:'n',15:'o',16:'p',17:'q',18:'r',19:'s',20:'t',21:'u',22:'v',23:'w',24:'x',25:'y',0:'z'}
    for j in range(sum):
        ans2[sum-1-j] = dict[ans[j]]
    print(''.join(ans2))


main()


Fixed submission:
def main():
    N = int(input())

    i = 1
    sum = 0
    temp = 0
    while True:
        temp = 0
        i += 1
        sum += 1
        for j in range(1,i):
            temp += 26**j

        if N <= temp:
            break

    #文字数はsum
    temp = 0
    for j in range(1,sum):
        temp += 26**j
    #print(N)
    N -= (temp+1)
    #print(N,sum)

    ans = []
    for j in range(sum):
        amari = N%26
        N = N//26
        #print(amari,flag)
        ans.append(amari)

    ans2 = [0]*len(ans)
    dict = {0:'a',1:'b',2:'c',3:'d',4:'e',5:'f',6:'g',7:'h',8:'i',9:'j',10:'k',11:'l',12:'m',13:'n',14:'o',15:'p',16:'q',17:'r',18:'s',19:'t',20:'u',21:'v',22:'w',23:'x',24:'y',25:'z'}
    for j in range(sum):
        ans2[sum-1-j] = dict[ans[j]]
    print(''.join(ans2))


main()

Buggy python submission (verdict: Runtime Error):
def main():
    N = int(input())

    i = 1
    sum = 0
    temp = 0
    while True:
        temp = 0
        i += 1
        sum += 1
        for j in range(1,i):
            temp += 26**j

        if N <= temp:
            break

    # 文字数はsum
    #for j in range(1,sum):
        #temp += 26**j
    #N -= temp

    ans = []
    flag = 0
    cnt = 0
    for j in range(sum):
        amari = N%26
        if amari == 0:
            flag = 1
            cnt += 1
            if cnt == 1:
                #print(amari,flag)
                ans.append(amari)
                N = N//26
                continue
        N = N//26
        #print(amari,flag)
        ans.append(amari-flag)

    #print(ans)

    ans2 = [0]*(sum)
    dict = {1:'a',2:'b',3:'c',4:'d',5:'e',6:'f',7:'g',8:'h',9:'i',10:'j',11:'k',12:'l',13:'m',14:'n',15:'o',16:'p',17:'q',18:'r',19:'s',20:'t',21:'u',22:'v',23:'w',24:'x',25:'y',0:'z'}
    for j in range(sum):
        ans2[sum-1-j] = dict[ans[j]]
    print(''.join(ans2))


main()


Fixed submission:
def main():
    N = int(input())

    i = 1
    sum = 0
    temp = 0
    while True:
        temp = 0
        i += 1
        sum += 1
        for j in range(1,i):
            temp += 26**j

        if N <= temp:
            break

    #文字数はsum
    temp = 0
    for j in range(1,sum):
        temp += 26**j
    #print(N)
    N -= (temp+1)
    #print(N,sum)

    ans = []
    for j in range(sum):
        amari = N%26
        N = N//26
        #print(amari,flag)
        ans.append(amari)

    ans2 = [0]*len(ans)
    dict = {0:'a',1:'b',2:'c',3:'d',4:'e',5:'f',6:'g',7:'h',8:'i',9:'j',10:'k',11:'l',12:'m',13:'n',14:'o',15:'p',16:'q',17:'r',18:'s',19:'t',20:'u',21:'v',22:'w',23:'x',24:'y',25:'z'}
    for j in range(sum):
        ans2[sum-1-j] = dict[ans[j]]
    print(''.join(ans2))


main()