Why do I have to initialize one of the stacks to -1? Where would top1 point to? 

```
int ar[SIZE];
int top1 = -1;
int top2 = SIZE;

void push_stack1 (int data)
{
  if (top1 < top2 - 1)
  {
    ar[++top1] = data;
  }
  else
  {
    printf ("Stack Full! Cannot Push\n");
  }
}

```