#include <avr/io.h>

int main(void) {
    char c;
    // 9600-7-E-1
    UCSRA = 0x00;
    UCSRB = 0x18;
    UCSRC = 0xA4;
    UBRRH = 0;
    UBRRL = 23;
    while (1) {
        while (~UCSRA & 1<<RXC);
        if (UCSRA & (1<<FE | 1<<DOR | 1<<PE)) {
            c = UDR;
            c = '?';
        }
        else {
            c = UDR;
        }
        while (~UCSRA & 1<<UDRE);
        UDR = c;
        while (~UCSRA & 1<<UDRE);
        UDR = c;
    }
    return 0;
}