初学C语言求指导

自学C语言有点简单的不懂，求指导

#include<stdio.h> 



int mul(int a, int b);
main()
{
	int a,b,c;
	a = 5;
	b = 10;
	c = mul(a,b);
	
	printf("multiplication of %d and %d is %d",a,b,c);
}
int mul(int x, int y);
int p;
{
	p = x*y;
	return p;
}

编译过后总是显示：“18 1 F:\源代码\子程序的使用.cpp [Error] expected unqualified-id before ‘{’ token”。请问是什么问题？