任给两个数，求两数相除之后，精确到小数点后一千万位。

class Program
{
static void Main(string[] args)
{
Divide(10, 3, 10000000);
}

/// <summary>
/// 除法运算
/// </summary>
/// <param name="a">被除数</param>
/// <param name="b">除数</param>
/// <param name="p">小数位数</param>
static string Divide(int a, int b, int p)
{
// 使用字符串（可变字符串，比 string 效率高）保存计算结果
System.Text.StringBuilder builder = new StringBuilder(p);
int count = 0;

builder.Append(a / b);
a = a % b;

if (a != 0)
builder.Append(".");

while (a != 0 && count < p)
{
a *= 10;

builder.Append(a / b);<