C programming increment operator related

Question

int a=3;
int b=++a + a++;
printf("%d",b);
gives output 9 whereas
int a;
scanf("%d",&a);
int b=++a + a++;
printf("%d",b);
gives output 8 for a=3
Why??

Answer 1

The produced answers are undefined behaviors ¹. The code is violating the sequencing rule ² by using the increment operator ³ , carelessly.

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¹ https://en.wikipedia.org/wiki/Undefined_behavior
² https://en.wikipedia.org/wiki/Sequence_point#Sequence_points_in_C_and_C++
³ https://en.cppreference.com/w/c/language/operator_incdec