什么意思（7 13 [Error] 'ADDxy' was not declared in this scope）

/计算两数之和/
#include <stdio.h>
int main()
{
int x,y,z;
scanf("%d%d",&x,&y);
z=ADDxy(x,y);
printf("%d\n",z);
return 0;
}
int ADDxy(int a,int b)
{
int c;
c=a+b;
return(c);
}

int ADDxy(int a,int b) 这个函数调用它之前 ，没声明，修改如下：

#include <stdio.h>
int ADDxy(int a,int b); //函数声明在前
int main()
{
   int x,y,z;
   scanf("%d%d",&x,&y);
   z=ADDxy(x,y);       //函数调用在后
   printf("%d\n",z);
   return 0;
}
int ADDxy(int a,int b)
{
   int c;
   c=a+b;
   return(c);
}