C語言，避免溢出，求兩數平均值

#include <stdio.h>
int main()
{
 double a=0.0,b=0.0;
 double c=0.0;
 double sum=0.0;
 scanf("%lf %lf",&a,&b);
 c=(a-b)/2.0;
 if (c<=0)
 {
  sum=b+c;
 }
 else
 {
  sum=c+b;
 }
 printf("%lf\n",sum);
 return 0;
}