#include <cstdlib>
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <map>
#include <stack>
#include <vector>
#include <map>
#include <queue>
#include <ctime>
#include <algorithm>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef unsigned int uint;
const int inf=1061109567;
const int maxn=1010;
const int maxm=10000010;
const int mod=1000000007;

int main()
{
    freopen("mountain.in","r",stdin);
    freopen("mountain.out","w",stdout);
    //clock_t st=clock();
    ll n,d,a,b;
    scanf("%lld%lld%lld%lld",&n,&d,&a,&b);
    ll ans=b-a+d*n;
    printf("%lld\n",ans/(2*d));
    //clock_t ed=clock();
    //printf("\nTime used : %.5lf Ms\n",double(ed-st)/CLOCKS_PER_SEC);
    return 0;
}