%{
#include <stdio.h>

int yylex(void);
void yyerror(char const *);
%}

%token ZERO
%token ONE

%%
input: {printf("Enter the line:\n");}
	| input line
	;

line: '\n' {printf("Empty line\n");}
    | error '\n' {yyerrok;}
    | exp3 '\n'
    | exp4 '\n'
    | exp3 exp1'\n'
    | exp4 exp2'\n'
    ;

exp1: ONE ONE
	| exp1 ONE ONE
	;

exp2: ZERO ZERO
	| exp2 ZERO ZERO
	;

exp3: exp1 exp2
	| exp3 exp1 exp2
	;

exp4: exp2 exp1
	| exp4 exp2 exp1
	;

%%
int yylex(void)
{
    int c;
    c=getchar();
    if(c=='1')
    return ONE;
    if(c=='0')
    return ZERO;
    if (c==EOF)
		return 0;
    return c;
}

void yyerror(char const *s)
{
	fprintf(stderr, "%s\n", s);
}