Write a C Program to verify the string ab*

#include<stdio.h>

#include<conio.h>

#include<string.h>

void main()

{

            char str[20];

            int flag,i;

            clrscr();

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

            gets(str);

            if(str[0]=='a')

            {

                        for(i=0;i<strlen(str);i++)

                        {

                                    if(str[i+1]=='b' || str[i+1]=='\0')

                                                flag=1;

                                    else

                                    {

                                                flag=0;

                                                break;

                                    }

                        }

                        if(flag==1)

                                    printf("\nString is Validddd.....");

                        else

                                    printf("\nInvalid String");

            }

            else

            {

                        printf("\nInvalid");

            }

            getch();

}