Thread: Mysterious linked list crashes

I have to currently build a program that removes the forth student from a circle until here are none left. I have done the following code however when running it crashes with no explanation of error. Any help would be grateful and any possible explanations of the error caused.

Code:

 
#include <stdio.h>
#include <stdlib.h>
int main()
    {
    /*Declarations*/
    int i,n,j;
    struct node
        {
        int num;
        struct node * next;
        };
    struct node *header, *last, *temp, *present,*previous, *first;      
    header = NULL;
    printf("Enter how many students in the circle\n");
    scanf("%d",&n);
    /*create the circle of students*/
    for ( i=1;i<=n;i++)
        {
        temp = malloc(sizeof(struct node));
        temp->num=i;
        if (i==1)
            header = temp;
        else
            last -> next = temp;
        last = temp;
        }
    /*prints of list and removes students*/
     present = header;
     first = present;
    
    while (n>0)
        {
        for (j=0;j<3;j++)
            {
            if (present == NULL)
                present = first;
            if (j==1)
                previous = present;
            present = present -> next;
            }
        if (present == first)
            first = first -> next;
        /*print off each student before removing them*/
        printf("%d ",present -> num);
        temp = present;
        previous->next = present -> next;
        free(temp);
        n--;
        }
    }

At the end of the while loop, you free(temp) and thereby make "present" point to freed memory. You should make present point to something that exists before the next iteration of the while loop.