Write a C program to verify the string (aa|bb)*

#include<stdio.h>

#include<conio.h>

#include<string.h>

void main()

{

          char str[20];

          int flag=0,i;

          clrscr();

          printf("\n Enter the String:-");

          gets(str);

          if(str[0]=='\0'||(str[0]=='a' && str[1]=='a') ||( str[0]=='b' && str[1]=='b'))

          {

                   for(i=2;i<strlen(str);i+=2)

                   {

                             if((str[i]=='a' || str[i+1]=='a'))

                                      flag=1;

                             if((str[i]=='b' || str[i+1]=='b'))

                                      flag=1;

                             else

                             {

                                      flag=0;

                                      goto Next;

                             }

                   }

          Next: if(flag==1)

                             printf("\n Validddd.....");

                   else

                             printf("\n INValid String");

          }

          else

          {

                   printf("\nInvalid");

          }

          getch();

}