Array Implementation of Queues

#include <stdio.h>
#include <stdlib.h>
int *a;
int count, size;//Initially the size of the queue is not initiallized
void enqueue(int value){
    a[count++] = value;
}
void dequeue(){
    int i;
    for(i = 0; i<count-1; i++) a[i] = a[i+1];//Overwriting
    count--;
}
void display(){
    int i;
    printf("The queue is ");
    if(count == 0) printf("empty.");
    else{
        if(count == size) printf("full.\n");
        for(i = 0; i < count; i++) printf("%d\t", a[i]);
    }
}
int main(){
    int value, choice;
    printf("Enter the size of the Queue: ");
    scanf("%d", &size);
    a = (int *) malloc(sizeof(int) *size);
    while(1){
        display();
        printf("\n\n1.Enqueue 2.Dequeue 3.Exit: ");
        scanf("%d", &choice);
        switch(choice){
            case 1:
                if(count < size){
                    printf("\nEnter a value to enqueue: ");
                    scanf("%d", &value);
                    enqueue(value);
                }else printf("Error: Overflow.");
                break;
            case 2:
                if(count > 0) dequeue();
                else printf("Error: Underflow.");
                break;
            case 3:
                return 0;
        }
    }
}