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Why ElGamal signature is insecure with nonrandom k?

In ElGamal signature scheme like:

keygen: $\beta=\alpha^a\ (mod\ p)$ where $\alpha$ and $p$ are large prime numbers. public key is $(p,\alpha,\beta)$ and secret key is $a$.

Sign: Choosing random $k$ that $gcd(k,p-1)=1$, calucalting $r:=a^k\ (mod\ p)$ and $s:=k^{-1}.(m-ar)\ (mod\ p-1)$ then we send $(m,r,s)$

Verify: calculating $v_1:=\beta^r.r^s (mod\ p)$ and $v_2:= \alpha^m (mod\ p)$ if $v_1$ and $v_2$ be equal we verify this sign.

In the booklet, I'm reading for cryptography course said if $k$ be non-random this signature scheme is insecure. what's the reason?