https://projecteuler.net/problem=430
期望线性性，递推数列
int main(){
LL n = 1e10, m = 4000; DB z = 0;
FOR_1(i, 1, n){
DB p = (2.*i*(n+1-i)-1)/((DB)n*n);
DB t = 1.-2*p;
if (sgn(t, 0.99) <= 0){
z += (n-2*(i-1))/2;
break;
}
z += (1.+pow(t, m));
}
OT(z);
}
