#include <bits/stdc++.h>

using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    int n, v, z, t;
    cin >> n >> v >> z >> t;
    int Done[n];
    int nowV = t * -1;
    for(int x = 0; x < n; x++)
    {
        Done[x] = nowV + v;
        nowV = Done[x];
        if(Done[x] < 0)
        {
            Done[x] = 0;
        }
    }
    int nowZ = 0, time = 0;
    for(int x = 0; x < n; x++)
    {
        time += max(Done[x] - nowZ, 0);
        nowZ = max(Done[x], nowZ) + z;
    }
    cout << time;
    return 0;
}