BZOJ2698染色

/*
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*/ 
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<cmath>
#define N 1000005
using namespace std;
double ans,p[N],tot;
int n,m,s,t;
int read()
{
    int x=0,f=1; char ch;
    while (ch=getchar(),ch<‘0‘||ch>‘9‘) if (ch==‘-‘) f=-1;
    while (x=x*10+ch-‘0‘,ch=getchar(),ch>=‘0‘&&ch<=‘9‘);
    return x*f;
} 
double fastpower(double x,int k)
{
    double res=1.0;
    for (; k; k>>=1,x=x*x)  if (k&1) res=res*x;
    return res;
}
double get(int x,int l,int r)
{
    return 1.0*(x-l+1+x-r+1)*(r-l+1)/2;
}
int main()
{
    n=read(); m=read(); s=read(); t=read();
    ans=0;
    tot=get(n,s,t);
    for (int i=s; i<=n; i++) p[i]+=get(i-1,s,min(i-1,t));
    for (int i=1; i<=n-s; i++) p[i]+=get(n-i,s,min(n-i,t));
    for (int i=1; i<=n; i++) ans+=1-fastpower(p[i]/tot,m);
    printf("%0.3lf",ans);
    return 0;
    
}