Queue using Linked Lists in C

Here's an implementation of a basic Linked Queue in C

	#include <stdio.h>
	#include <stdlib.h>
	// Just use a singly linked list with f pointed to front of the queue
	// and r pointing to rear of the queue
	struct queue {
	int data;
	struct queue *next;
	};
	struct queue *f, *r;
	void enqueue(int num) {
	struct queue *nn;
	nn = (struct queue *)malloc(sizeof(struct queue));
	nn -> data = num;
	nn -> next = NULL;
	if(f == NULL) {
	f = r = nn;
	} else {
	r -> next = nn;
	r = nn;
	}
	}
	int dequeue() {
	int num;
	struct queue *temp;
	if(f == NULL) {
	return -1;
	} else {
	num = f -> data;
	temp = f;
	f = f -> next;
	free(temp);
	return num;
	}
	}
	void display() {
	struct queue *p;
	if(f == NULL) {
	printf("Queue is Empty\n");
	} else {
	p = f;
	printf("The elements of the queue are: \n");
	while(p != NULL) {
	printf("%d ", p -> data);
	p = p -> next;
	}
	printf("\n");
	}
	}
	int main()
	{
	int num, choice, rpt = 1;
	while(rpt != 0) {
	printf("1. Enqueue\n2. Dequeue\n3. Display\n4. Exit\n");
	printf("---------------\n");
	printf("Enter a choice: ");
	scanf("%d", &choice);
	switch(choice) {
	case 1:
	printf("Enter a num: ");
	scanf("%d", &num);
	enqueue(num);
	break;
	case 2:
	num = dequeue();
	if(num != -1) {
	printf("%d deleted from queue\n", num);
	} else {
	printf("Queue Underflow\n");
	}
	break;
	case 3:
	display();
	break;
	case 4:
	exit(0);
	}
	printf("Do you want to continue? (1/0): ");
	scanf("%d", &rpt);
	system("cls");
	}
	return 0;
	}