using namespace std;
const int N=1000;
int n,s;
double dp[N+5][N+5];
double dfs(int i,int j){
	if(i==n&&j==s) return 0;
	if(dp[i][j]!=0) return dp[i][j];
	dp[i][j]=1;
	if(i+1<=n) dp[i][j]+=dfs(i+1,j)*(n-i)*1.0*j/(n*s);
	if(j+1<=s) dp[i][j]+=dfs(i,j+1)*i*(s-j)/(1.0*n*s);
	if(i+1<=n&&j+1<=s) dp[i][j]+=dfs(i+1,j+1)*(n-i)*1.0*(s-j)/(n*s);
	dp[i][j]/=(1.0-i*j*1.0/(n*s));
	return dp[i][j];
}
int main(){
	scanf("%d%d",&n,&s);
	printf("%.4lf\n",dfs(0,0));
	return 0;
}