this is my solution and it works

#include < iostream >

using namespace std;

int main()
{
    int a, b, div, rem;
    cin >> a >> b;
    int sum = a;
    while(a >= b)
    {
        div = a / b;
        sum += div;
        rem = a % b;
        a = div + rem;
    }
    cout << sum << endl;
    return 0;
}

[link to the problem] (http://codeforces.com/problemset/problem/379/A)
`

but I saw other people solutions and they have used a formula that calculates answer without any loop

 ans = (a * b — 1)/(b — 1) 

Can someone please help me how to get this formula?