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 with problems in programming contests, we recommend you to try some problems in “AtCoder Beginners Selection” (https://atcoder.jp/contests/abs).

This problem can be solved using a for statement. Specifically, the answer can be found by the following procedure:

• Prepare a variable \(ans\) initialized with \(0\).
• For each \(i=1,2,\ldots,N\), add \((-1)^i\times i^3\) to \(ans\).
• Now \(ans\) stores the sought answer; print the value of \(ans\).

The problem can be solved by appropriately implementing the procedure above.

Sample code (Python3)

N = int(input())
ans = 0
for i in range(1, N + 1):
    if i % 2 == 0:
        ans += i * i * i
    else:
        ans -= i * i * i
print(ans)

Sample code (C++)

#include <bits/stdc++.h>
using namespace std;
int main() {
	int N;
	cin >> N;
	int ans = 0;
	for (int i = 1; i <= N; i++) {
		if (i % 2 == 0) {
			ans += i * i * i;
		} else {
			ans -= i * i * i;
		}
	}
	cout << ans << endl;
}

Alternatively, you can use the formula \(\displaystyle \sum_{i=1}^N (-1)^i\times i^3=\frac{(-1)^N(4N^3+6N^2-1)+1}8\).

Sample code (Python3)

N = int(input())
print(((-1) ** N * (4 * N**3 + 6 * N**2 - 1) + 1) // 8)