#include <iostream>
#include <set>
using namespace std;
long long n;
int main() {
cin >> n;
long long s = 0;
long long e = 1000000000000000000LL;
while (s+1 < e) {
long long k = (s+e)/2;
long long tmpn = n;
long long cnt = 0;
while (tmpn > 0) {
if (tmpn > k) {
tmpn -= k;
cnt +=k;
tmpn = tmpn - (tmpn/10);
} else {
cnt += tmpn;
tmpn = 0;
}
}
if (cnt >= n/2) {
e = k;
} else {
s = k;
}
}
cout << e << endl;
// your code goes here
return 0;
}
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