编写函数double fun(double x, int m),它的功能是将浮点数X保留M个小数，第M+1位四舍五入，如输入123.456，保留两位小数输出123.46-The preparation of function double fun (double x, int m), its function is to float X retained a small number of M, the first M+1 bit rounded, such as the importation