if you use Alcinoe TalJsonDocument it's will be simple as this :
aJsonDoc := TalJsonDocU.create;
aJsonDoc.loadFromFile(...);
for i := 0 to aJsonDoc.node.childnodes.count-1 do begin
myValue := aJsonDoc.node.childNodes[i].getchildNodeValueText(['b', 'email']);
if myValue <> '' then break;
end;