def convert_part_to_lowercase(s, idx) :
length = len(s)
if (idx < 0 or idx > length): return s
s = list(s)
for i in range(length) :
if (i >= idx and (ord(s[i]) >= 65 and ord(s[i]) <= 90)):
s[i] = chr(ord(s[i]) + 32)
return "".join(s)
s = "PYTHON PROGRAMMING"
s = convert_part_to_lowercase(s, 4)
print(s)
'''
run:
PYTHon programming
'''