Language: C

// /*
//   GOOD is not Enough
//   You've got to be GREAT.
// */
// #include<bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// // #include<boost/multiprecision/cpp_int.hpp>
// // using namespace boost::multiprecision;
// using namespace __gnu_pbds;
// using namespace std;
// #define int long long
// #define mod 1000000007
// #define FAST ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
// #define f(i,n) for(int i=0;i<n;i++)
// #define fp(i,k,n) for(int i=k;i<=n;i++)
// #define fr(i,k,n) for(int i=k;i>=n;i--)
// #define pb push_back
// #define pii pair<int,int>
// #define dbg(x) cout << (#x) << " is " << (x) << '\n' 
// #define F first
// #define S second
// #define all(a) a.begin(),a.end()
// typedef tree<int,null_type,less_equal<int>,rb_tree_tag,tree_order_statistics_node_update>ordered_set;
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
 
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
 
using namespace std;
using namespace __gnu_pbds;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;
 
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
 
typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;
 
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
 
#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
 
#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
int MOD = 1000000007;
const ll INF = 1e18;
const int MX = 100001; //check the limits, dummy

ll modExp(ll base, ll power) {
    if (power == 0) {
        return 1;
    } else {
        ll cur = modExp(base, power / 2); cur = cur * cur; cur = cur % MOD;
        if (power % 2 == 1) cur = cur * base;
        cur = cur % MOD;
        return cur;
    }
}

ll inv(ll base) {
    int g = MOD, r = base, x = 0, y = 1;
    while (r != 0) {
        int q = g / r;
        g %= r; swap(g, r);
        x -= q * y; swap(x, y);
    }
    return x < 0 ? x+MOD : x;
}

ll mul(ll A, ll B) {
	return (A*B)%MOD;
}

ll add(ll A, ll B) {
	return (A+B)%MOD;
}

ll dvd(ll A, ll B) {
    return mul(A, inv(B));
}

ll sub(ll A, ll B) {
    return (A-B+MOD)%MOD;
}
int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);    
    int N;  int P; P=2019;
    string S; cin >> S;
    N=S.length();

    ll counts[P]; F0R(i, P) counts[i] = 0;
    counts[0] = 1;
    int cur = 0;
    MOD = P;
    F0R(i, N) {
        cur *= 10; cur += S[i] - '0'; cur %= MOD;
        int val = modExp(10, N-1-i) * cur; val %= MOD;
        counts[val]++;
    }

    ll ans = 0;
    F0R(i, P) ans += (counts[i] * (counts[i] - 1)) / 2;

    cout << ans << endl;
	
	return 0;
}



Compiler output:
main.c:27:10: fatal error: bits/stdc++.h: No such file or directory
   27 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.