Elevator Trouble

#include <iostream>
using namespace std;

typedef long long ll;

#define MAX 9999999999999LL

ll f,s,g,u,d;

bool ok(ll k1, ll k2) {
    if (k1 < 0 || k2 < 0) return false;
    if ( k1 != 0 && k2 != 0) {
        return !(s-d <= 0 && s+u > f);
    }
    return true;
}

int main() {
    cin >> f >> s >> g >> u >> d;
    ll ans = MAX;

    for (ll k1 = 0; k1 < f; ++k1) {
        ll delta = g - (k1 * u + s);
        if (delta == 0) {
            if (k1 < ans) ans = k1;
        } else if (d != 0 && delta < 0LL && (-delta % d == 0) && ok (k1, -delta / d)) {
            ll k2 = -delta / d;
            if (k1 + k2 < ans) {
                ans = k1+k2;
            }
        }
    }

    if (ans == MAX) {
        cout << "use the stairs" << endl;
    } else {
        cout << ans << endl;
    }
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED