Where to get libraries for esp8266wifi.h and wficlientsecure.h?

Using Arduino Programming Questions
1

When i use the above header files....i get error as
C:\Users\Viswa\AppData\Local\Temp\arduino_modified_sketch_503525\sketch_sep02a.ino:4:25: fatal error: ESP8266WiFi.h: No such file or directory

#include <ESP8266WiFi.h>

could you provide me a solution?

2

Installing with Boards Manager

3

A Beginner's Guide to the ESP8266

Pieter