You do some weird things. No need for the semi colons to add more endl's. Can just keep inserting them into the stream. Not sure why you're using doubles? And your for loop is strange. Why do you have d involved in the loop at all? It doesn't do anything but just add 1 to itself. All your program does is print startum through n, one at a time.
Could anyone by any chance help me to write
if/while error for both cin >>.
Basically, if user enters non-number there should be a "Wrong number!"
And then it jumps to cout << " Continue (1) End (0)?" << endl; part.