#include <cstdio>

int main() {
    int n;
    scanf("%d", &n);
    double area = (double)n * n / 16.0;
    printf("%.2f\n", area);
    return 0;
}
/**************************************************************
	Problem: 1420
	User: huangcanwu
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:1144 kb
****************************************************************/