Stack using Linked Lists in C

This is a simple Linked Representation of Stacks in C.
I didn't like the one given in Reema Thareja's book so I made a simpler one.

	#include <stdio.h>
	#include <stdlib.h>
	struct stack {
	int data;
	struct stack *next;
	};
	struct stack *top = NULL;
	void push(int num) {
	struct stack *nn;
	nn = (struct stack *)malloc(sizeof(struct stack));
	nn -> data = num;
	if(top == NULL) {
	nn -> next = NULL;
	top = nn;
	} else {
	// Insert at beginning
	nn -> next = top;
	top = nn;
	}
	}
	int pop() {
	int num;
	struct stack *temp;
	if(top == NULL) {
	return -1;
	} else {
	num = top -> data;
	temp = top;
	top = top -> next;
	free(temp);
	return num;
	}
	}
	int peek(){
	if(top == NULL) {
	return -1;
	} else {
	return top -> data;
	}
	}
	void display(){
	struct stack *p;
	if(top == NULL) {
	printf("Stack is Empty\n");
	} else {
	p = top;
	printf("The elements in the stack are: \n");
	while(p != NULL) {
	printf("%d\n", p -> data);
	p = p -> next;
	}
	}
	}
	int main()
	{
	int rpt = 1, num, choice;
	while(rpt != 0) {
	printf("1. Push\n");
	printf("2. Pop\n");
	printf("3. Peek\n");
	printf("4. Display\n");
	printf("5. Exit\n");
	printf("--------------\n");
	printf("Enter choice: ");
	scanf("%d", &choice);
	switch(choice) {
	case 1:
	printf("Enter number: ");
	scanf("%d", &num);
	push(num);
	break;
	case 2:
	num = pop();
	if(num != -1) {
	printf("%d deleted from stack\n", num);
	} else {
	printf("Stack Underflow\n");
	}
	break;
	case 3:
	num = peek();
	if(num != -1) {
	printf("Element on top of the stack = %d\n", num);
	} else {
	printf("Stack Underflow\n");
	}
	break;
	case 4:
	display();
	break;
	case 5:
	exit(0);
	}
	printf("Do you want to continue? (1/0): ");
	scanf("%d", &rpt);
	system("cls");
	}
	return 0;
	}