编写程序，计算下面公式并输出结果。



要求：（1）编写一个函数计算n!

（2）编写主函数，由键盘输入n和m，调用（1）中的函数完成计算。

（3）输入n和m要给出提示，并检查n和m的合理性，不合理的输入应输出错误信息，并不再进行计算。-Write programs to calculate the following formula and output. Requirements: (1) Write a function to calculate n! (2) the preparation of the main function, the keyboard input n and m, call (1) complete the calculation of the function. (3) input n and m be given prompt and check the reasonableness of n and m, unreasonable input should output an error message, and no longer calculated.