Matter++

w = int(input())
string = input()

l, r = 0, 1
min = 99999999
en = 1
minstring = string
letters = {string[0] : 1}

while r <= len(string):
    if en < w:
        if string[r] in letters:
            amount = letters[string[r]]
            en -= amount**2
            letters[string[r]] += 1
            en += (amount+ 1)**2
        else:
            letters[string[r]] = 1
            en += 1
        r += 1
    elif en == w:
        minstring = string[l:r]
        break
    else:
        amount = letters[string[l]]
        en -= amount**2
        letters[string[l]] -= 1
        en += (amount-1)**2
        l += 1
    if en < min and en >= w: 
        min = en
        minstring = string[l:r]

print(minstring)

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: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: ACCEPTED

Test 18

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 25

Verdict: ACCEPTED

Test 26

Verdict: WRONG ANSWER

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 31

Verdict: ACCEPTED

Test 32

Verdict: WRONG ANSWER

Test 33

Verdict: ACCEPTED

Test 34

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 35

Verdict: WRONG ANSWER

Test 36

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 37

Verdict: WRONG ANSWER

Test 38

Verdict: ACCEPTED

Test 39

Verdict: ACCEPTED

Test 40

Verdict: WRONG ANSWER

Test 41

Verdict: ACCEPTED

Test 42

Verdict: WRONG ANSWER

Test 43

Verdict: WRONG ANSWER

Test 44

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 45

Verdict: WRONG ANSWER

Test 46

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 47

Verdict: WRONG ANSWER

Test 48

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 49

Verdict: ACCEPTED

Test 50

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 51

Verdict: WRONG ANSWER

Test 52

Verdict: WRONG ANSWER

Test 53

Verdict: WRONG ANSWER

Test 54

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 55

Verdict: WRONG ANSWER

Test 56

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 57

Verdict: WRONG ANSWER

Test 58

Verdict: WRONG ANSWER

Test 59

Verdict: ACCEPTED

Test 60

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 61

Verdict: WRONG ANSWER

Test 62

Verdict: WRONG ANSWER

Test 63

Verdict: WRONG ANSWER

Test 64

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 65

Verdict: WRONG ANSWER

Test 66

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 67

Verdict: WRONG ANSWER

Test 68

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 69

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 70

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 12, in <module>
    if str...

Test 71

Verdict: WRONG ANSWER