#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef long double ld;

// O(n/k), special treatment with k = 1.

ll n,m,k;
ll ans = 0;

ll f(ll x){
    if(x > n/k || (n - x*k > m*(k-1)) || x < 0)return -1;
    ll result = x*n - k * (x*(x-1))/2;
    ll n2 = n - x*k;
    ll k2 = n2/m + 1;
    ll x2 = n2 - m*(n2/m);
    result += x2*n2 - k2 * (x2*(x2-1))/2;
    ll n3 = n2 - x2*k2;
    ll k3 = k2-1;
    ll x3 = m-x2;
    result += x3*n3 - k3 * (x3*(x3-1))/2;
    ans = max(result, ans);
    return result;
}


int main(){
    cin >> n >> m >> k;
    if(k == 1){
        cout << (n*n+n)/2 << "\n";
        return 0;
    }
    ans = 0;

    for(ll x = 0; x <= n/k; x++){
        ans = max(ans, f(x));
    }

    cout << ans << "\n";
    return 0;
}