Hacker Rank Recursive Digit Sum

#include<bits/stdc++.h>

using namespace std;

#define fast ios_base::sync_with_stdio(0)
#define bfast cin.tie(0)
#define outs(x) cout << x << " "
#define outn(x) cout << x << "\n"
#define sf scanf
#define pf printf
#define pfn(x , k) printf(k , x)
#define nl puts("")
#define psb push_back
#define mset(c,v) memset(c , v , sizeof c)
#define loop0(n) for(int i=0; i<n; i++)
#define loop1(n) for(int i=1; i<=n; i++)
#define mpair(x , y) make_pair(x , y)
#define all(x) x.begin(), x.end()
#define pi acos(-1.0)
#define psb push_back
#define clr clear()

typedef unsigned long long ull;
typedef long long LL;
typedef vector<int>vii;
typedef vector<LL>vll;
typedef vector<string>vs;
typedef map<int, int>mpii;
typedef map<string, int>mpsi;
typedef map<char, int>mpci;
typedef map<LL, LL>mpll;

const int mod = 1000007;
const int high = 1002;

LL digit(LL n, LL tmp)
{
    if(n == 0) return tmp;

    tmp += (n % 10);

    return digit(n/=10 , tmp);
}

int main()
{
    fast;
    string s;
    int k ;
    while(cin >> s >> k)
    {
        LL sum = 0;

        for(LL i=0; i<s.length(); i++)
        {
            sum+=(s[i] - 48);
        }

        LL p = sum * k;

        while(p >= 10)
        {
            p = digit(p, 0);
        }

        cout << p << "\n";
    }

    return 0;
}