# include <stdio.h>

/* t_0 = 1 */
/* t_1 = 3 */
/* t_2 = 2 */
/* t_3 = 1 */
/* pour i >= 4 on a t_i = t_(i-1) + t_(i-2) +t_(i-3) + t_(i-4)  */

/* lire en entier n et afficher t_n */
int main(void) {
  int i, n;
  int t0, t1, t2, t3, w;
  /* initialisation */
  t0 = 1; /* t0 = t_0 */
  t1 = 3; /* t1 = t_1 */
  t2 = 2; /* t2 = t_2 */
  t3 = 1; /* t3 = t_3 */
  /* saisie contrôlée de n */
  do {
  printf("entrer un entier positif ou nul\n");
  scanf("%d",&n);
  } while (n < 0); 
  /* n >= 0  cas particuliers */
  switch (n) {
  case 0 : 
      printf("pour n = 0 on a t_0 = %d\n", t0); 
      break;
  case 1:  
      printf("pour n = 1 on a t_1 = %d\n", t1); 
      break;
  case 2 : 
      printf("pour n = 2 on a t_2 = %d\n", t2); 
      break;
  case 3 : 
      printf("pour n = 3 on a t_3 = %d\n", t3); 
      break;
  }
  for (i = 4; i <= n; i++) {
    w = t3 + t2 + t1 + t0; 
    /* w = t_i  t3=t_(i-1) t2= t_(i-2) t1 =t_(i-3) t0= t_(i-4)*/
    t0 = t1;
    t1 = t2;
    t2 = t3;
    t3 = w;
  }
  printf("pour n = %d on a t_n = %d\n", n, w);

  return 0;
}