#include <iostream>
#include <vector>
#include <climits>
#include <algorithm>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <bitset>
#include <utility>
#include <numeric>
#include <queue>
#include <stack>

using ll = long long;
using namespace std;

constexpr int MOD = 1e9 + 7;
constexpr ll MOD_LL = ll(1e9) + 7;

int main(void) {
	double p;
	cin >> p;
	
	double ans = 10.0 * p - 4.0;
	
	if( ans <= 0.0 ) {
		printf("%.10f\n", 0.0);
	} else {
		printf("%.10f\n", ans);
	}
	
	return 0;
}