If you are new to learning programming and do not know where to start, please try Problem A “Welcome to AtCoder” from practice contest. There you can find a sample code for each language.
Also, if you are not familiar to problems in programming contests, we recommend you to try some problems in “AtCoder Beginners Selection” (https://atcoder.jp/contests/abs).
「競プロ典型 90 問」(Typical 90 Problems of Competitive Programming) (https://atcoder.jp/contests/typical90) is a collection of typical 90 competitive programming problems; unfortunately, currently the problem statements are all Japanese.
「C++入門 AtCoder Programming Guide for beginners (APG4b)」(https://atcoder.jp/contests/APG4b) is a C++ tutorial for competitive programmers. Sadly, this is only in Japanese too.

This problem can be solved by receiving the input as an integer value, calculating just as described in the problem statement, and outputting the answer.

Depending on languages and implementations you choose, you may have to be careful of overflows and outputting format.

Sample code (C)

#include<stdio.h>
#include<math.h>

int main(){
	double h;
	scanf("%lf",&h);
	printf("%.9f\n",sqrt(h*(12800000+h)));
}

Sample code (Python)

import math
H=int(input())
print(math.sqrt(H*(12800000+H)))