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#include <bits/stdc++.h>
#define all(vec) vec.begin(),vec.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define getFro(vec) (vec.empty()?0:vec.front())
#define getBac(vec) (vec.empty()?0:vec.front())
#define debug(var) cerr << #var <<":"<<var<<"\n";
#define DEBUG(variable) \
do { \
std::cerr << #variable << ":"; \
for (const auto& elem : variable) { \
std::cerr << elem << " "; \
} \
std::cerr << "\n"; \
} while (0)
#define uniVec(var) \
do { \
sort(var.begin(),var.end());\
var.resize(unique(var.begin(),var.end())-var.begin());\
} while (0)
#define debugN(var,N) \
do{ \
std::cerr<<#var<<":"; \
FOR(i,1,N){ \
std::cerr<<var[i]<<" "; \
} \
std::cerr<<"\n"; \
}while(0)
#define debugMap(variable) \
do { \
std::cerr << #variable << ":\n"; \
for (const auto& pair : variable) { \
std::cerr << " " << pair.first << " => " << pair.second << "\n"; \
} \
std::cerr << std::endl; \
} while (0)
#define lson(var) (var<<1)
#define rson(var) ((var<<1)+1)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef __int128 i128;
typedef pair<ll,ll> pll;
typedef array<ll,3> arr;
typedef double DB;
typedef long double LD;
typedef pair<DB,DB> pdd;
typedef pair<ll,bool> plb;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(1e18)+3;
constexpr ll mod=static_cast<ll>(1e9)+7;
constexpr double eps=1e-8;
ll N,M,K,T;
// 已经找到了i种bug且在j个子系统里发现了bug，离完成目标还剩多少天数。
DB dp[1010][1010];
/*
*/
inline void solve(){
ll s;
cin>>N>>s;
DB ns=N*s;
ROF(i,N,0){
ROF(j,s,0){
if(i==N && j==s){
dp[N][s]=0.0;
}else{
// 从i+1转移过来，说明bug种类数量增加了，增长的bug不属于原来有的bug
dp[i][j]=(dp[i+1][j]*(N-i)*j+dp[i][j+1]*(s-j)*i+
// dp
dp[i+1][j+1]*(N-i)*(s-j)+ns) /(ns-i*j);
}
}
}
#ifdef LOCAL
FOR(i,0,N){
FOR(j,0,s){
debug(dp[i][j]);
}
}
#endif
cout<<setprecision(4)<<fixed<<dp[0][0]<<"\n";
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
// cin>>T;
// while(T--){
// solve();
// }
solve();
return 0;
}
/*
WA http://bailian.openjudge.cn/practice/solution/49203195/
*/
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