Runtime Erorr

#include <cstdio>
struct node
{
	int data;
	node *next;
};
int main ()
{
	int n,m;
	scanf ("%d%d",&n,&m);
	node *head,*p,*now,*prev;
	now = head;
	for (int i = 2;i <= n;i++)
	{
		p = new node;
		p -> data = i;
		p -> next = NULL;
		now = p;
	}
	now -> next = head;
	now = head;
	prev = head;
	while ((n--) > 1)
	{
		for (int i = 1;i <= m;i++)
		{
			prev = now;
			now = now -> next;
		}
		printf ("%d",now -> data);
		prev -> next = now -> next;
		delete now;
		now = prev -> next;
	}
	printf ("%d",now -> data);
	delete now;
	return 0;
}