Number Theory Prove a complete square 1,11,111

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I have to prove that every number in the series 11,111,1111 ... Is not a complete square, Can you give me a clue how to do this?

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(I think $1$ can be discarded, because actually $1=1^2$...)

Hint: Consider the numbers in modulo 4. Then compare with $x^2 \mod 4$.