#include <iostream>
#include <vector>
using namespace std;
const int M = 10, len = 1;
struct BigInt{
vector<int> a;
void operator = (int k){
a.clear();
do{
a.push_back(k % M), k /= M;
}
while (k);
}
void operator += (BigInt X){
BigInt res;
for (int i=0, c = 0;i<max(a.size(), X.a.size()) or c > 0;i++){
if (i < a.size())
c += a[i];
if (i < X.a.size())
c += X.a[i];
res.a.push_back(c % M), c /= M;
}
swap(a, res.a);
}
void operator -= (BigInt X){
BigInt res;
for (int i=0, fl = 0;i<a.size();i++){
int c = 0;
if (i < X.a.size())
c += X.a[i];
a[i] -= fl, fl = 0;
a[i] -= c;
if (a[i] < 0)
a[i] += M, fl = 1;
}
}
void operator *= (BigInt X){
BigInt res;
while (res.a.size() < a.size() + X.a.size())
res.a.push_back(0);
for (int i=0;i<a.size();i++){
for (int j=0;j<X.a.size();j++)
res.a[i+j] += a[i] * X.a[j] % M, res.a[i+j+1] += (a[i] * X.a[j]) / M;
}
for (int i=0;i+1<res.a.size();i++)
res.a[i+1] += res.a[i] / M, res.a[i] %= M;
while (res.a.size() > 1 and res.a.back() == 0)
res.a.pop_back();
swap(a, res.a);
}
void operator /= (int two){
BigInt res;
for (int i=a.size() - 1, cr = 0;i + 1; i--){
cr = cr * M + a[i];
if (cr / 2 != 0 or res.a.size() > 0)
res.a.insert(res.a.begin(), cr / 2), cr %= 2;
}
if (res.a.size() == 0)
res.a.push_back(0);
swap(a, res.a);
}
bool operator < (BigInt X){
if (a.size() != X.a.size())
return a.size() < X.a.size();
for (int i=a.size()-1;i>=0;i--)
if (a[i] != X.a[i])
return a[i] < X.a[i];
return 0;
}
};
void print(BigInt X){
for (int i=X.a.size();i;i--){
string s = to_string(X.a[i-1]);
while (s.size() < len)
s = '0' + s;
cout<<s;
}
cout<<'\n';
}
int main(){
BigInt N, one, two, ten, d, l, r, mid, A;
one = 1, two = 2, ten = 10, N = 0;
string s;
cin>>s;
for (auto i : s)
d = i - '0', N *= ten, N += d;
l = 0, r = N, N *= two;
while (1){
mid = l, mid += r, mid /= 2;
if (l.a == mid.a)
break;
A = mid, A += one, A *= mid;
if (A < N)
l = mid;
else
r = mid;
}
N -= r;
print(N);
}
