Undefined reference to `yylval' using Flex and Bison

How can I resolve the "undefined reference to 'yylval'" error in the simplest program ever?

This is my lexer.l file

%{
    #include "parser.tab.h"
%}

%%
[0-9]+      {
                yylval = atoi(yytext);
                return NUMBER; }
"-"         {return MINUS;}

%%

int main ()
{
     yylex();
     return 0;
}

and this is my parser file

%{
#include <stdio.h>
#include <stdlib.h>

void yyerror(const char* msg); 
int n=0;
%}

%token MINUS 
%token NUMBER

%%

program :   NUMBER MINUS NUMBER { n=$1-$3 ; printf("%d\n",n);}

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

int main() {
    printf("Enter  ");
    yyparse();  // Start parsing
    return 0;
}

And these are the commands I run to build this program:

C:\Users\User\Desktop\workbench\prevodiocip\nnn>bison -d parser.y

C:\Users\User\Desktop\workbench\prevodiocip\nnn>flex lexer.l

C:\Users\User\Desktop\workbench\prevodiocip\nnn>gcc -o parser.tab.c lex.yy.c -lfl
C:\Users\User\AppData\Local\Temp\ccOvWXJo.o:lex.yy.c:(.text+0x1b1): undefined reference to `yylval'
collect2.exe: error: ld returned 1 exit status

I have tried everything, but nothing works for me.