f1_ptr = fopen("prime","r");
if(f1_ptr == NULL)
{
printf("Can not open file1");
exit(-1);
}
f2_ptr = fopen("other","r");
if(f2_ptr == NULL)
{
printf("Can not open file2");
exit(-1);
}
f3_ptr = fopen("result","w");
if(f3_ptr == NULL)
{
printf("Can not open file3");
exit(-1);
}