Matter++

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

l, r = 0, 1
min = 99999999
en = 1
minstring = ""

while r <= len(string):
    if en < w:
        r += 1
    elif en == w:
        minstring = string[l:r]
        break
    else:
        l += 1

    letters = []
    substring = string[l:r]
    en = 0

    for i in range(len(string[l:r])):
        if substring[i] not in letters:
            en += substring.count(substring[i]) ** 2
            letters.append(substring[i])

    if en < min and en >= w: 
        min = en
        minstring = substring

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: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: WRONG ANSWER

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: WRONG ANSWER

Test 33

Verdict: ACCEPTED

Test 34

Verdict: ACCEPTED

Test 35

Verdict: WRONG ANSWER

Test 36

Verdict: WRONG ANSWER

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: ACCEPTED

Test 45

Verdict: WRONG ANSWER

Test 46

Verdict: ACCEPTED

Test 47

Verdict: WRONG ANSWER

Test 48

Verdict: WRONG ANSWER

Test 49

Verdict: ACCEPTED

Test 50

Verdict: ACCEPTED

Test 51

Verdict: WRONG ANSWER

Test 52

Verdict: WRONG ANSWER

Test 53

Verdict: WRONG ANSWER

Test 54

Verdict: ACCEPTED

Test 55

Verdict: WRONG ANSWER

Test 56

Verdict: ACCEPTED

Test 57

Verdict: WRONG ANSWER

Test 58

Verdict: WRONG ANSWER

Test 59

Verdict: ACCEPTED

Test 60

Verdict: ACCEPTED

Test 61

Verdict: WRONG ANSWER

Test 62

Verdict: TIME LIMIT EXCEEDED

Test 63

Verdict: TIME LIMIT EXCEEDED

Test 64

Verdict: TIME LIMIT EXCEEDED

Test 65

Verdict: TIME LIMIT EXCEEDED

Test 66

Verdict: TIME LIMIT EXCEEDED

Test 67

Verdict: TIME LIMIT EXCEEDED

Test 68

Verdict: TIME LIMIT EXCEEDED

Test 69

Verdict: TIME LIMIT EXCEEDED

Test 70

Verdict: TIME LIMIT EXCEEDED

Test 71

Verdict: TIME LIMIT EXCEEDED