#include <cstdlib>
#include <iostream>
using namespace std;
int main()
{
double e;
cin >> e;
double a = 1.0, b = 25.0;
double r=0, y=0;
do{
r+=y;
y=1.0/(a+b);
a *= 4;
b *= 5;
}while( y>e );
cout << r << endl;
}
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