#include <stdio.h>

int main(void) {
  int grade7, grade13, legal13;
  
  /* Prompt for input */
  printf("Enter Danish 13 scale grade: ");
  scanf("%d", &grade13);

  /* Find the corresponding 7 step grade */
  legal13 = 1;
  switch(grade13){
    case 13: case 11: 
      grade7 = 12; break;
    case 10: 
      grade7 = 10; break;
    case 9: case 8: 
      grade7 = 7; break;
    case 7: 
      grade7 = 4; break;
    case 6: 
      grade7 = 2; break;
    case 5: case 3: 
      grade7 = 0; break;
    case 0: 
      grade7 = -3; break;
    default: 
      legal13 = 0;
  }

  /* Print the result */
  if (legal13)
    printf("The 13 scale grade %d corresponds "  
           "to the 7 step grade %d\n",
           grade13, grade7);
  else
    printf("The 13 scale grade %d is illegal\n");

  
  return 0;
}