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

// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
// #define ordered_set tree<int, null_type,less_equal<int>, rb_tree_tag,tree_order_statistics_node_update>

template <typename T> istream& operator >> (istream& in, vector<T>& v) { for (auto &it:v) in >> it; return in; }
template <typename T> ostream& operator << (ostream& os, const vector<T>& v) { for (auto &it:v) os << it << " "; return os; }
template <typename T1, typename T2> istream& operator >> (istream& in, pair<T1,T2>& p) { cin >> p.first >> p.second; return in; }
template <typename T1, typename T2> ostream& operator << (ostream& os, const pair<T1,T2>& p) { cout << p.first << " " << p.second; return os; }
template <typename T1, typename T2> void maxn(T1& a, T2 b) { a = max(a,b); }
template <typename T1, typename T2> void maxx(T1& a, T2 b) { a = max(a,b); }

#define int long long int
#define ll int
#define double long double
#define pb push_back
#define ff first
#define ss second
#define all(v) v.begin(),v.end()
#define allr(v) v.rbegin(),v.rend()
#define sz(v) (int)v.size()
#define deb(x) cout<<#x<<"="<<x<<endl;
#define pii pair<int,int>
#define vi vector<int>

const int mod = 1e9+7;
const int mod2 = 998244353;
const double PI = 3.1415926535897932384626433832795;
const int INF = 0xFFFFFFFFFFFFFFFLL;

int find_cycle(int d,int b,int t,int p,int mid)
{
    int buffer_size=mid;
    int time_to_fill=(buffer_size+d-1)/d;
    int curr_consumption_time=buffer_size/(b-d);
    int den=(time_to_fill+curr_consumption_time)*d;
    int num=t*b;
    int curr_cycles=(num+den-1)/den;
    return curr_cycles;
}

void solve()
{
    int d,b,t,p; cin>>d>>b>>t>>p;
    int tot_bytes=b*t;
    int consumption_time=d*p/(b-d);
    int den=(p+consumption_time)*d;
    int tot_cycles=(tot_bytes+den-1)/den;
    int l=0,r=p*d+10;
    while(l+1<r)
    {
        int mid=(l+r)>>1;
        int curr_cycles=find_cycle(d,b,t,p,mid);
        if(curr_cycles>tot_cycles) l=mid;
        else r=mid;
    }
    cout<<r<<endl;
}

int32_t main() {
    // #ifndef ONLINE_JUDGE
    freopen("video.in", "r", stdin);
    freopen("video.out", "w", stdout);
    // #endif
    cin.tie(0)->sync_with_stdio(0);
    int tc=1;
    // cin>>tc;
    while(tc--) solve();
    return 0;
}