B. Arithmetic Exercise

#include <bits/stdc++.h>

using namespace std;
using ii  = pair<int , int>;
using iii = pair<int , ii>;
using vi  = vector< int >;
using vii = vector< ii >;
using ll  = long long;
using vl  = vector< ll >;
using pll = pair<ll , ll>;
using vll = vector< pll >;
using plll= pair<ll , pll>;

#define FastIO cin.tie(0);cout.tie(0);ios::sync_with_stdio(0);
#define LSOne(a) ( (a) & -(a) )
#define INF  ll(4e18)
#define OO   int(1e9)
#define EPS      1e-9
#define PB  push_back
#define S      second
#define F       first

const int N = 2e5 + 5e1;
const ll MOD = 1e9 + 7;
char buff[N];

ll _pow(ll n ,ll p)
{
    ll x = 1;
    while( p > 0 )
    {
        if( p & 1 ) x = (x * n);
        n = (n * n);
        p >>= 1;
    }

    return x;
}

int main()
{
    ll a ,b ,k;
    scanf("%lld%lld%lld" ,&a ,&b ,&k);

    auto bits = [&](ll x) -> ll {
        ll cnt = 0;
        while( x ) x /= 10 ,cnt ++;
        return cnt;
    };

    if( a == b )
    {
        printf("1.");
        while( k-- ) printf("0");
        return 0;
    }

    printf("0.");
    a *= 10;

    vl ans;
    while( k )
    {
        ll n = bits(a);
        bool flag = false;
        for(ll i = n - 1 ; i >= 0 ; i--)
        {
            ll a1 = a / _pow(10ll ,i);

            if( a1 >= b )
            {
                flag = true;
                ll c = a1 / b;
                ll d = c * b;
                k -= 1;
                ans.PB(c);
                a -= d;
                break;
            }
        }

        if( !flag )
        {
            a *= 10;
            if( a < b )
            {
                k -= 1;
                ans.PB(0);
            }
        }
    }

    int m = (int)ans.size();
    while( (int)ans.size() != m + 1 )
    {
        ll n = bits(a);
        bool flag = false;
        for(ll i = n - 1 ; i >= 0 ; i--)
        {
            ll a1 = a / _pow(10ll ,i);

            if( a1 >= b )
            {
                flag = true;
                ll c = a1 / b;
                ll d = c * b;
                k -= 1;
                ans.PB(c);
                a -= d;
                break;
            }
        }

        if( !flag )
        {
            a *= 10;
            if( a < b )
            {
                k -= 1;
                ans.PB(0);
            }
        }
    }

    if( ans.back() >= 5 )
    {
        for(int i = (int)ans.size() - 2 ; i >= 0 ; i--)
        {
            if( ans[i] == 9 ){
                ans[i] = 0;
            } else {
                ans[i] += 1;
                break;
            }
        }
    }

    for(int i = 0 ; i < (int)ans.size() - 1 ; i++){
        printf("%lld" ,ans[i]);
    }
    puts("");
    return 0;
}