#include<bits/stdc++.h>#defineintlonglongusingnamespace std;constint maxn =3e5+5;int n, m, k;int c[maxn];voidinit(){
cin >> n >> k;for(int i =1; i <= k; i++){
cin >> c[i];
m += c[i];}}voidsolve(){double ans =0;for(int i =1; i <= k; i++){
ans +=(double)(c[i]*(c[i]-1))/((m)*(m -1));}
ans =(double)(1- ans)*(n -1);
cout << fixed <<setprecision(3)<< ans <<"\n";}signedmain(){// ios::sync_with_stdio(0);// cin.tie(0);int t =1;//cin >> t;while(t--){init();solve();}}