Merkkijono

x = int(input())
debug = False
aakkoset = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o",
            "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z", "å", "ä", "ö"]

parilliset = list(range(0,500,2))

merkkijono = []

for a in range(x):
    if a == 0:
        merkkijono.append("a")
    else:
        for b in range(len(merkkijono)):
            merkkijono.insert(parilliset[b], aakkoset[a])
    if debug: print(f'DEBUG ~ {a} | {"".join(merkkijono)}')

merkkijono_loppu = merkkijono
merkkijono_loppu.reverse()
merkkijono = merkkijono + merkkijono_loppu
merkkijono.remove("a")
print("".join(merkkijono))

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 11

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 12

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 13

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 14

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 15

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...

Test 16

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 15, in <module>
    merkki...