/** made by amunduzbaev **/
#include <bits/stdc++.h>
using namespace std;
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define ub upper_bound
#define lb lower_bound
#define ll long long
#define ld long double
#define pii pair<int, int>
#define pll pair<ll, ll>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(),x.rend()
#define fastios ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define vll vector<ll>
#define vii vector<int>
const int N = 305;
const int mod = 1e9+7;
const ll inf = 1e18;
const ld Pi = acos(-1);
ll n, m, lsz, k, last, szz, timer;
bool ok;
int sss(ll last){
ll res = 0;
while(last){ res++, last /= 10; }
return res;
}
void fun(ll prev, ll nw){
while(prev < nw){
szz += sss(prev);
prev += 2;
}
}
void solve(){
fastios
cin>>n;
ll i=2;
last = 1, szz = 1;
ll prevl = 0;
while(szz < n){
prevl = last;
last += (i*2)-1;
szz += i;
i++;
}
if(szz == n) cout<<last%10<<endl;
else{
for(;szz > n && prevl < last; last -= 2) szz --;
cout<<last<<endl;
}
return;
}
int main(){
fastios
int t = 1;
if(t) solve();
else {
cin>>t;
while (t--) solve();
}
return 0;
}
