Language: C++

// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define vrep(v, i) for(int i = 0; i < (v).size(); i++)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
#define dump(a, n) for(int i = 0; i < n; i++)cout << a[i] << "\n "[i + 1 != n];
#define dump2(a, n, m) for(int i = 0; i < n; i++)for(int j = 0; j < m; j++)cout << a[i][j] << "\n "[j + 1 != m];
#define bit(n) (1LL<<(n))
#define INT(n) int n; cin >> n;
#define LL(n) ll n; cin >> n;
#define DOUBLE(n) double n; cin >> n;
using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef complex<double> cd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << p.first << " " << p.second; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << itr->first << ":" << itr->second; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}


template<typename T>
T extgcd(T a, T b, T &x, T &y){
  T d = a;
  if(b != 0){
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  }else{
    x = 1, y = 0;
  }
  return d;
}

template<typename T>
T gcd(T a, T b) {
  if(a > b)return gcd(b, a);
  if(a == 0)return b;
  return gcd(b % a, a);
}

template <typename T>
T modinv(T a, T m){
  long long x = 0, y = 0;
  extgcd<long long>(a, m, x, y);
  x %= m;
  if(x < 0)x += m;
  return x;
}

template <typename T>
T power(T a, T n, T mod) {
  T res = 1;
  T tmp = n;
  T curr = a;
  while(tmp){
    if(tmp % 2 == 1){
      res = (T)((ll)res * curr % mod);
    }
    curr = (T)((ll)curr * curr % mod);
    tmp >>= 1;
  }
  return res;
}

ll ten[19];
ll l[21], r[21];

template<typename T>
pair<T, T> chinese_reminder_theorem(vector<T> b, vector<T> m){
  T r = 0, M = 1;
  for(int i = 0; i < b.size(); i++){
    T x, y;
    T d = extgcd<T>(M, m[i], x, y);
    if((b[i] - r) % d != 0)return make_pair(0, -1);
    T tmp = (b[i] - r) / d * x % (m[i] / d);
    r += M * tmp;
    M *= m[i] / d;
  }
  r %= M;
  if(r < 0)r += M;
  return make_pair(r % M, M);
}

template <typename T>
long long garner(vector<T> b, vector<T> m, T MOD){
  m.emplace_back(MOD);
  vector<long long> coef(m.size(), 1);
  vector<long long> consts(m.size(), 0);
  for(int i = 0; i < b.size(); i++){
    long long t = ((b[i] - consts[i]) % m[i]) * modinv<long long>(coef[i], m[i]) % m[i];
    for(int j = i + 1; j < m.size(); j++){
      consts[j] = (consts[j] + t * coef[j] % m[j]) % m[j];
      coef[j] = coef[j] * m[i] % m[j];
    }
  }
  return consts.back();
}

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(20);
  ten[0] = 1;
  FOR(i, 1, 19)ten[i] = ten[i-1] * 10;
  LL(L); LL(A); LL(B); LL(M);
  FOR(i, 1, 19){
    l[i] = (ten[i-1] - A + B - 1) / B;
    r[i] = (ten[i] - A + B - 1) / B;
    l[i] = max(0LL, l[i]);
    r[i] = min(L, r[i]);
  }
  ll X = M;
  ll M = mod;
  ll res = 0;
  ll curr = 1;
  A %= M;
  B %= M;
  for(int i = 18; i >= 1; i--){
    ten[i] %= M;
    if(l[i] >= L)continue;
    if(r[i] <= l[i])continue;
    ll tmp1 = ((A + B * (l[i] % M) % M) % M) * power<ll>(ten[i], r[i] - l[i], M) % M;
    ll tmp2 = B * (power<ll>(ten[i], r[i] - l[i] - 1, M) - 1) % M;
    tmp2 = tmp2 * ten[i] % M;
    tmp2 = tmp2 / ((ten[i] - 1 + M) % M) % M;
    if(tmp2 < 0)tmp2 += M;
    ll tmp3 = (A + B * ((r[i] % M) - 1) % M) % M;
    ll tmp = (tmp1 + tmp2 - tmp3) % M;
    if(tmp < 0)tmp += M;
    tmp = tmp / ((ten[i] - 1 + M) % M) % M;
    if(tmp < 0)tmp += M;
    res = (res + curr * tmp % M) % M;
    curr = curr * power<ll>(ten[i] % M, (r[i] - l[i]), M) % M;
  }
  if(res < 0)res += M;
  vector<ll> b(1) = {res};
  vector<ll> m(1) = {mod};
  cout << garner(b, m, X) << endl;
  return 0;
}



Compiler output:
a.cc: In function 'int main(int, const char**)':
a.cc:149:6: error: redeclaration of 'long long int M'
  149 |   ll M = mod;
      |      ^
a.cc:141:27: note: 'long long int M' previously declared here
  141 |   LL(L); LL(A); LL(B); LL(M);
      |                           ^
a.cc:24:18: note: in definition of macro 'LL'
   24 | #define LL(n) ll n; cin >> n;
      |                  ^
a.cc:172:19: error: expected ',' or ';' before '=' token
  172 |   vector<ll> b(1) = {res};
      |                   ^
a.cc:173:19: error: expected ',' or ';' before '=' token
  173 |   vector<ll> m(1) = {mod};
      |                   ^