和tyvj的Easy一样吧（然而还是不会2333）
期望是不能直接平方的（涨姿势），所以，，呵呵
1 #include<bits/stdc++.h> 2 #define inf 0x7fffffff 3 #define LL long long 4 #define N 100005 5 using namespace std; 6 inline int ra() 7 { 8 int x=0,f=1; char ch=getchar(); 9 while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();} 10 while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();} 11 return x*f; 12 } 13 double f[N],l[N],ll[N],a[N]; 14 int main() 15 { 16 int n=ra(); 17 for (int i=1; i<=n; i++) 18 { 19 scanf("%lf",&a[i]); 20 l[i]=(l[i-1]+1)*a[i]; 21 ll[i]=(ll[i-1]+l[i-1]*2+1)*a[i]; 22 f[i]=f[i-1]+(3*l[i-1]+3*ll[i-1]+1)*a[i]; 23 } 24 printf("%.1lf",f[n]); 25 return 0; 26 }