But ,
is a solution to the PCP. Hence the same string of terminals 
has two derivations. Both these derivations are, clearly, leftmost. Hence G is ambiguous.
If It is important to note that any string of terminals cannot have more than one derivation in 
and 
Because, every terminal string which are derivable under these grammars ends with a sequence of integers 
This sequence uniquely determines which productions must be used at every step of the derivation.
Hence, if a terminal string, 
,
has two leftmost derivations, then one of them must begin with the step.
and thus continues with derivation under ,
and the other must begin with the step 
and then continues with derivations under .