input = require('fs').readFileSync('/dev/stdin', 'utf8')
lines = input.split('\n')
N = lines.shift() * 1
lines.pop()
c = lines.map(x => x.split(' ').map(y => y * 1))
d = []
r = 1
p = 2
for (i=0;i<N-1;i++) {
    for (j=i+1;j<N;j++) {
        d.push(Math.sqrt(Math.pow(c[i][0] - c[j][0], 2) + Math.pow(c[i][1] - c[j][1], 2)))
    }
    r *= i + 2
    p *= i + 1
}
sum = 0
for (i=0;i<d.length;i++) {
    sum += d[i] * p
}
console.log((sum / r).toFixed(10))