Coding challenge optimization - c

Question

I am participating in a coding challenge that consistes in the following:
We start with a number(size of the string) followed by a string (ex: 5WWFAE) by stdin.
The letters given represent elements, F-fire, W-water, E-earth, A-air. Fire and Water cancel each other and Earth and Air cancel as well.
The goal is to simplify the string by canceling the elements and print it, but you can only cancel adjacent members, for example: "7AFEAWWE" would result in "AWE".
In this challenge the points are given depending on how fast the program runs and how little memory you spend, somehow some people still have more points than me. Could you help-me optimising it?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define stack_is_empty() stack_index == -1
#define stack_push() stack[++stack_index] = c
#define stack_pop() --stack_index
#define can_join() (stack[stack_index] + c == 'F' + 'W') || (stack[stack_index] + c == 'A' + 'E')
void process_string(int len)
{
char c;
char *stack = malloc(len * sizeof(char));
register int stack_index = -1;
register int i = 0;
for (i = 0; i < len; ++i)
{
scanf("%c", &c);
if (stack_is_empty())
{
stack_push();
}
else if (can_join())
{
stack_pop();
}
else
{
stack_push();
}
}
printf("%*.*s\n", stack_index + 1, stack_index + 1, stack);
}
int main()
{
int len;
//* Input:
scanf("%d", &len);
if (len == 0)
{
printf("\n");
return 0;
}
process_string(len);
return 0;
}