单选错位

小学数学题......

#include<cstdio>
#include<cstring>
#include<iostream>
#define mem(a,b) memset(a,b,sizeof(a))
#define dd double
#define ll long long
using namespace std;
int minn(int a,int b){return a<b?a:b;}
const int N=10000100;
const int M=1000000100;

int n,A,B,C;
int a[N];
double ans;

int main(){
    scanf("%d%d%d%d%d",&n,&A,&B,&C,a+1);
    for(int i=2;i<=n;++i)
      a[i]=((ll)a[i-1]*A+B)%100000001;
    for(int i=1;i<=n;++i)
      a[i]=a[i]%C+1;
    
    for(int i=1;i<n;++i)
        ans+=minn(a[i],a[i+1])*((1.0/(dd)a[i])*(1.0/(dd)a[i+1]));
    ans+=minn(a[1],a[n])*((1.0/(dd)a[1])*(1.0/(dd)a[n]));
    printf("%.3lf",ans);
    //while(1);
    return 0;
}