E - Σ[k=0..10^100]floor(X／10^k)

• Time : O(n)
• Space : O(1)

#include <bits/stdc++.h>

#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")//Comment optimisations for interactive problems (use endl)
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

struct PairHash {
    inline std::size_t operator()(const std::pair<int, int> &v) const { return v.first * 31 + v.second; }
};

#define M_PI 3.14159265358979323846  /* pi */
#define MAX_N 202020
#define INF 987654321
#define EPS 1e-9
#define ll long long
#define pll pair<ll, ll>
#define vpll vector<pll>
#define vall3 vector<array<ll,3>>
#define all3 array<ll,3>
#define all5 array<ll,5>
#define vall5 vector<all5>
#define vll vector<ll>
#define vi vector<int>
#define vs vector<string>
#define usll unordered_set<ll>
#define uspll unordered_set<pll, PairHash>
#define umll unordered_map<ll,ll>
#define vvs vector<vs>
#define vvll vector<vll>
#define all(a) begin(a), end(a)
#define rall(a) rbegin(a), rend(a)

ll __gcd(ll x, ll y) { return !y ? x : __gcd(y, x % y); }

using namespace std;

string solve(string& s) {
    string res = "";
    ll sum = accumulate(all(s), 0ll, [](ll sum, char ch) {
       return sum + (ch & 0b1111);
    }), carry = 0;

    while(!s.empty()) {
        ll pop = s.back() & 0b1111; s.pop_back();
        ll now = sum + carry;
        carry = now / 10;
        now %= 10;
        res.push_back(now | 0b110000);
        sum -= pop;
    }

    reverse(all(res));
    return carry ? to_string(carry) + res : res;
}

int main() {
    ios_base::sync_with_stdio(0);
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.precision(12);
    ll tc = 1;
    //cin>>tc;
    for (ll i = 1; i <= tc; i++) {
        string s;
        cin>>s;
        cout << solve(s) << endl;
    }
    return 0;
}