#include <cstdio> #include <algorithm> using namespace std; class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n; public:range(int n):i({0}),n({n}){}range(int i,int n):i({i}),n({n}){}I& begin(){return i;}I& end(){return n;}}; double E[111][111][111]; int main(void) { int n; scanf("%d", &n); for(int k0 : range(n+1)) { for(int k1 : range(n+1)) { for(int k2 : range(n+1)) { int k = k0 + k1 + k2; if(k == 0) { continue; } E[k0][k1][k2] = double(n) / k; if(k0 > 0) { E[k0][k1][k2] += E[k0-1][k1+1][k2] * k0 / k; } if(k1 > 0) { E[k0][k1][k2] += E[k0] [k1-1][k2+1] * k1 / k; } if(k2 > 0) { E[k0][k1][k2] += E[k0] [k1] [k2-1] * k2 / k; } } } } vector<int> x(3); for(int i : range(n)) { int a; scanf("%d", &a); if(a < 3) { x[a]++; } } printf("%f\n", E[x[0]][x[1]][x[2]]); return 0; }