codeforces Educational Codeforces Round 21 B. Average Sleep Time

可以先把每一天的贡献次数乘上睡眠时间，最后加在一起除以（n-k+1）即可

#include<iostream>
#include<cstdio>
#include<cstring>
#include<ctime>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<set>
#include<bitset>
#include<map>
#include<tr1/unordered_map>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#define INF 0x3f3f3f3f
#define inf 2*0x3f3f3f3f
#define llinf 1000000000000000000
#define pi acos(-1.0)
#define mod 1000000007
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define lrt rt<<1
#define rrt rt<<1|1
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define per(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define mem(a,b) memset(a,b,sizeof(a))
#define lb(x) (x&-x)
#define gi(x) scanf("%d",&x)
#define gi2(x,y) scanf("%d%d",&x,&y)
#define gll(x) scanf("%lld",&x)
#define gll2(x,y) scanf("%lld%lld",&x,&y)
#define gc(x) scanf("%c",&x)
#define gc2(x,y) scanf("%c%c",&x,&y)
using namespace std;
using namespace std::tr1;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int>P;
/***********************************************/
ll n,k,a[200005];
double sum=0;
int main()
{
    cin>>n>>k;
    rep(i,0,n){gi(a[i]);}
    if(n+1>=2*k)rep(i,0,n)sum=sum+a[i]*min(min(k,(ll)(i+1)),(ll)n-i);
    else rep(i,0,n)sum=sum+a[i]*min(min(n-k+1,(ll)(i+1)),(ll)n-i);
    printf("%.10lf\n",sum/(n-k+1));
    return 0;
}