#include <stdio.h>

int main(void)
{
    /* declare integers */
    int grade_ten, grade;

    /* take input */
    printf("Enter numerical grade: ");
    scanf("%d", &grade);

    /* calculate tens digit */
    grade_ten = grade / 10;

    /* check if grade is > 100 or < 0 */
    if (grade_ten == 10 && ((grade % 10) >= 1) || grade_ten > 10 || grade < 0) {
                  printf("Invalid grade");
                  grade_ten = -2;
                  }
                  else {
                       grade;
                       }

    /* find letter grade */
    switch (grade_ten) {
           case 9: case 10:
                printf("Letter grade: A");
                break;
           case 8:
                printf("Letter grade: B");
                break;
           case 7:
                printf("Letter grade: C");
                break;
           case 6:
                printf("Letter grade: D");
                break;
           case 5: case 4: case 3: case 2: case 1: case 0:
                printf("Letter grade F");
                break;
                }

                return 0;
}