C Program Logic Question

#define f(g1,g2) g1##g2
main()
{
int var12=100;
printf("%d",f(var,12));
}

this prints the output 100. and if i change the value in f(var,12) as f(var,13) a error is coming and if i delete # then also error is coming i cant understand give a brief explanation

#define f(g1,g2) g1##g2

main()
{
int var12=100;
printf("%d",f(var,12));
}

this prints the output 100. and if i change the value in f(var,12) as f(var,13) a error is coming and if i delete # then also error is coming I cant understand give a brief explanation

## is String concatenation operator. This will concatenates two values in that Macro.
Here Var12=100.

In Macro f(g1,g2) is g1##g2.

So If f(var,12) then it is var12 so Answer is 100.
If we put f(var,13) then it is var13, which is not declared.