def op(x, y):
    return x + y
class SegTree:
    def __init__(self, init_val, op, ide_ele):
        n = len(init_val)
        self.n = n
        self.op = op
        self.ide_ele = ide_ele
        self.num = 1 << (n - 1).bit_length()
        self.tree = [ide_ele] * 2 * self.num
        for i in range(n):
            self.tree[self.num + i] = init_val[i]
        for i in range(self.num - 1, 0, -1):
            self.tree[i] = self.op(self.tree[2 * i], self.tree[2 * i + 1])

    def update(self, k, x):
        k += self.num
        self.tree[k] = x
        while k > 1:
            self.tree[k >> 1] = self.op(self.tree[k], self.tree[k ^ 1])
            k >>= 1

    def query(self, l, r):
        res = self.ide_ele

        l += self.num
        r += self.num
        while l < r:
            if l & 1:
                res = self.op(res, self.tree[l])
                l += 1
            if r & 1:
                res = self.op(res, self.tree[r - 1])
            l >>= 1
            r >>= 1
        return res

    def __getitem__(self, n):
        return self.tree[self.num+n]
    
    def List(self):
        return self.tree[self.num:self.num+self.n]

def compress(A):
    S = sorted(set(A))
    D = dict()
    for i in range(len(S)):
        D[S[i]] = i
    return D

N = int(input())
XY = sorted([list(map(int, input().split())) for _ in range(N)], key=lambda x:x[0])

P, Q = 0, 0
D = compress([xy[1] for xy in XY])
cnt = [0]*len(D)
for i in range(N):
    X, Y = XY[i]
    cnt[D[Y]] += 1
seg = SegTree(cnt, op, 0)
tmp = []
idx = 0
while idx < N:
    while idx+1 < N and XY[idx][0] == XY[idx+1][0]:
        tmp.append(XY[idx][1])
        seg.update(D[XY[idx][1]], seg[D[XY[idx][1]]]-1)
        idx += 1
    tmp.append(XY[idx][1])
    seg.update(D[XY[idx][1]], seg[D[XY[idx][1]]]-1)
    idx += 1
    while tmp:
        y = tmp.pop()
        if 1 <= D[y]:
            Q += seg.query(0, D[y])
        if D[y] < len(D)-1:
            P += seg.query(D[y]+1, len(D))

R, S = N*(N-1)//2, N*(N-1)//2
DX = compress([xy[0] for xy in XY])
cntX = [0]*len(DX)
for i in range(N):
    X, Y = XY[i]
    cntX[DX[X]] += 1
for c in cntX:
    if c >= 2:
        R -= c*(c-1)//2
for c in cnt:
    if c >= 2:
        S -= c*(c-1)//2

print((P-Q)/((R*S)**0.5))