失礼します。switch文をif文に書き換える問題なのですが、default文がうまくいきません。
（入力値が正しくてもprintfが実行されてしまします）

#include <stdio.h>
int main (void){
int grade;
int aCount = 0, bCount = 0, cCount = 0, dCount = 0, fCount = 0;
printf("試験結果の成績(A,B,C,D,F)を入力してください。\n");
printf("入力終了は1\n");
while ((grade = getchar()) != '1'){
if (grade == 'a' || grade == 'A')
++aCount;
if (grade == 'b' || grade == 'B')
++bCount;
if (grade == 'c' || grade == 'C')
++cCount;
if (grade == 'd' || grade == 'D')
++dCount;
if (grade == 'f' || grade == 'F')
++fCount;
if(!((grade == 'a' || grade == 'A') || (grade == 'b' || grade == 'B') || (grade == 'c' || grade == 'C') || (grade == 'd' || grade == 'D') || (grade == 'f' || grade == 'F')))
printf("入力値が異常です、再入力\n"); }
printf("\n成績結果\n");
printf("A: %d\n",aCount);
printf("B: %d\n",bCount);
printf("C: %d\n",cCount);
printf("D: %d\n",dCount);
printf("F: %d\n",fCount);
return (0);}
よろしくお願いします