函数接口定义：
struct complex multiply(struct complex x, struct complex y);
其中struct complex是复数结构体，其定义如下：
struct complex{
int real;
int imag;
};
裁判测试程序样例：
#include <stdio.h>
struct complex{
int real;
int imag;
};
struct complex multiply(struct complex x, struct complex y);
int main()
{
struct complex product, x, y;
scanf("%d%d%d%d", &x.real, &x.imag, &y.real, &y.imag);
product = multiply(x, y);
printf("(%d+%di) * (%d+%di) = %d + %di\n",
x.real, x.imag, y.real, y.imag, product.real, product.imag);
return 0;
}
/* 你的代码将被嵌在这里 */
输入样例：
3 4 5 6
输出样例：
(3+4i) * (5+6i) = -9 + 38i

struct complex multiply(struct complex x, struct complex y)
{ 
  struct complex product;//!!!!!!!!!!!!!
  product.real=x.real*y.real-x.imag*y.imag;
  product.imag=x.imag*y.real+y.imag*x.real;
  return product;
}