Problem:
Score: 400 points

Problem Statement

AtCoder Inc. has decided to lock the door of its office with a 3-digit PIN code.

The company has an N-digit lucky number, S. Takahashi, the president, will erase N-3 digits from S and concatenate the remaining 3 digits without changing the order to set the PIN code.

How many different PIN codes can he set this way?

Both the lucky number and the PIN code may begin with a 0.

Constraints

4 \leq N \leq 30000

S is a string of length N consisting of digits.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of different PIN codes Takahashi can set.

Sample Input 1

4
0224

Sample Output 1

3

Takahashi has the following options:

Erase the first digit of S and set 224.

Erase the second digit of S and set 024.

Erase the third digit of S and set 024.

Erase the fourth digit of S and set 022.

Thus, he can set three different PIN codes: 022, 024, and 224.

Sample Input 2

6
123123

Sample Output 2

17

Sample Input 3

19
3141592653589793238

Sample Output 3

329

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

/* atcoder */
#include "bits/stdc++.h"
using namespace std;

/* typedef */
/* long系 */
typedef long long ll;   // long long
typedef long double ld; // long double
/* vector系 */
typedef vector<int> vi;                    // intのvector
typedef vector<double> vd;                 // doubleのvector
typedef vector<ll> vll;                    // llのvector
typedef vector<ld> vld;                    // ldのvector
typedef vector<vector<int>> vvi;           // intの2次元vector
typedef vector<vector<double>> vvd;        // doubleの2次元vector
typedef vector<vector<ll>> vvll;           // llの2次元vector
typedef vector<vector<ld>> vvld;           // ldの2次元vector
typedef vector<pair<int, int>> vi_i;       // i_iのvector
typedef vector<pair<double, double>> vd_d; // d_dのvector
typedef vector<pair<ll, ll>> vl_l;         // l_lのvector
typedef vector<pair<ld, ld>> vld_ld;       // ld_ldのvector
typedef vector<string> vs;                 // stringのvector
/* pair系 */
typedef pair<int, int> i_i;       // int同士のpair
typedef pair<double, double> d_d; // double同士のpair
typedef pair<ll, ll> l_l;         // ll同士のpair
typedef pair<ld, ld> ld_ld;       // ld同士のpair

/* define */
#define all(v) v.begin(), v.end() // コンテナ全体
#define PI (acos(-1))             // 円周率pi

/* stringstream */
stringstream ss;
stringstream init_ss; // save default formatting

/* upper, lower */
struct ToUpper
{
    char operator()(char c)
    {
        return toupper(c);
    }
};
struct ToLower
{
    char operator()(char c)
    {
        return tolower(c);
    }
};

/* main */
void Main(void)
{
    ll n;
    cin >> n;
    string s;
    cin >> s;
    ll ans = 0;
    for (ll i = 0; i < 1000; i++)
    {
        string pin;
        ss.str("");                      // バッファクリア
        ss.clear(stringstream::goodbit); // 状態クリア
        ss.copyfmt(init_ss);             // 書式クリア
        ss << setw(3) << setfill('0') << i;
        ss >> pin;
        ll k = 0;
        for (ll j = 0;; j++)
        {
            if (k == 3)
            {
                ans++;
                break;
            }
            if (j == n)
                break;
            if (pin[k] == s[j])
                k++;
        }
    }
    cout << ans << endl;
}

int main(int argc, char const *argv[])
{
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    Main();

    return 0;
}