How to get the part of the string?

In additional:
[Quoted]  I need to use retrieved numbers for calculating.  So such sql doesn't work
a= 'abcdef';

 Select to_number(substr(a,instr(a,'&')+1)) from dual;

When '&' is missed, I get an error.

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		In additional:
		 I need to use retrieved numbers for calculating.
		 So such sql doesn't work
		a= 'abcdef';

		 Select to_number(substr(a,instr(a,'&')+1)) from dual;

		When '&' is missed, I get an error.