求高手指点一下

#include<Stdio.h>
double m;
double fun(double a,double b)
{
    m=a-b;
    return a+b;
}
main()
{
    double x,y;
    scanf("%lf,%lf",&x,&y);
    printf("%f\n",fun(x,y));
    printf("%f\n",m);
}