BZOJ 2134 单选错位

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 10000050
using namespace std;
int n,A,B,C,a[maxn];
double ans=0;
int main()
{
    scanf("%d%d%d%d%d",&n,&A,&B,&C,&a[1]);
    for (register int i=2;i<=n;i++) a[i]=((long long)a[i-1]*A+B)%100000001;
    for (register int i=1;i<=n;i++) a[i]=a[i]%C+1;
    for (register int i=1;i<=n-1;i++) ans+=1.0/max(a[i],a[i+1]);
    ans+=1.0/max(a[1],a[n]);
    printf("%.3lf\n",ans);
    return 0;
}