c - getchar() が -1 を返すのはなぜですか?

Question

次のコードで、 と の値がch2にch3等しいのはなぜ-1ですか?

char ch1;
char ch2;
char ch3;
printf("put Type: ");
ch1 = getchar();
_flushall();
printf("put Type: ");
ch2 = getchar();
_flushall();
printf("put Type: ");
ch3 = getchar();
printf("\n");
printf("the ascii value for this three types is: %d, %d, %d", ch1, ch2, ch3);