对话框程序，获得双重的值，可以更容易的计算。-Having doubled this value, we now have to add in the remainders and

the digits already computed by earlier steps.　If we did not overflow

in the previous step, we might still cause an overflow here.　That

will happen whenever the maximum value expressible in two digits, less

the amount we have to add, is greater than the result of the previous

step.