extern 用来改变变量作用域，如下该怎么改

#include<stdio.h>
int main()
{int n=81,m=72;
int f(int n,int m);
int g(int n,int m);
printf("%d %d",f(n,m),g(n,m));}

int f(int n,int m)
{int a,b,c,t;
if(a%b==c)
a=t;
b=a;
c=b;
return (a);
}

int g(int n,int m)
{int d,e,f;
extern int a;
f=d*e/a;
return (f);}