s = "kapkapkap"
 
def find_repeating_substring(s):
    # Проверим, что длина строки четная
    if len(s) % 2 != 0:
        return ""
 
    n = len(s)
 
    for i in range(1, n // 2 + 1):
        if n % i == 0:  # Длина подстроки должна делиться на общую длину без остатка
            substring = s[:i]
 
            is_valid = True
            for j in range(i, n, i):
                if s[j:j+i] != substring:
                    is_valid = False
                    break
 
            if is_valid:
                return substring
 
    return "No"  # Подходящей подстроки нет
 
print(find_repeating_substring(s))

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