Checking if the key is in the dict

When adding and accessing elements to dictionaries, we might accidentally try to access a key that hasn’t been added to the dictionary yet. In that case, Python will tell us that a KeyError occurred and there isn’t such a key in the dictionary. To check if the key is in the dictionary, we can use the in keyword (exactly as we would do in lists, tuples, or sets).
costs = {
    'Living room': 200,
    'Bathroom': 400,
    'Kitchen': 500,
    'Balcony': 100,
}
print(costs['Bedroom'])   # KeyError: 'Bedroom'

if 'Bedroom' in costs:
    print(costs['Bedroom'])
else:
    costs['Bedroom'] = 150

print(costs)
# {'Living room': 200, 'Bathroom': 400, 'Kitchen': 500, 'Balcony': 100, 'Bedroom': 150}

Challenge

You are asked to calculate the most frequent word in the essay and output it. You know that words are separated by a space and that you should ignore uppercase/lowercase difference. It is guaranteed that there is only a single most common word.
The input contains a single line of text, where words are separated by a space.
The output should contain the most frequent word of the input.
Input
Output
Bob is a great person! He is an engineer.
is
Anna is awesome, call Anna
anna

Constraints

Time limit: 2 seconds

Memory limit: 512 MB

Output limit: 1 MB

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