Codeforces Round #516 (Div. 1, by Moscow Team Olympiad) D

#include <bits/stdc++.h>
#define LL long long
using namespace std;
LL n,x,k,lb,ub,l,r,ans=-1;
int main() {
    scanf("%lld%lld%lld%lld",&n,&l,&r,&k);
    x=(r>=l?r-l+1:r+n-l+1)%n;
    for(LL i=k,j,r;i;i=j){
        j=k/(k/i+1);r=k/i;
        ub=min(2*n,i);
        lb=max(n,j+1);
        ub=min(ub,min((2*n+k-2*x+1)/(r+1),(k-x)/r));
        lb=max(lb,(k-2*x+r-1)/r);
        if(ub>=lb)ans=max(ans,ub);
    }
    if(x<=k&&k<=x*2)ans=max(ans,n+min(k-x,x-1)+n-x+1);
    if(!x&&k==n)ans=max(ans,n+1);
    printf("%lld\n",~ans?ans-n:ans);
}