PKU 2096

#define ll long long
#include <iostream>
using namespace std;

const int OO = 1e9;
const double EPS = 1e-9;

int n,s;
double mem[1001][1001];

double solve(int cn, int cs) {
    if(cn == 0 && cs == 0) {
        return 0;
    }
    if(mem[cn][cs] != -1)
        return mem[cn][cs];
    double pn = (1.0*cn)/n;
    double ps = (1.0*cs)/s;
    double &ret = mem[cn][cs];
    ret = 0;
    if(cn > 0 && cs > 0) {
        ret += pn*ps*(1 + solve(cn-1,cs-1));
    }
    if(cn > 0) {
        ret += pn*(1-ps)*(1+solve(cn-1,cs));
    }
    if(cs > 0) {
        ret += (1-pn)*ps*(1+solve(cn,cs-1));
    }
    ret += (1-pn)*(1-ps);
    ret /= (1-(1-pn)*(1-ps));
    return ret;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    cin >> n >> s;
    for(int i = 0; i <= n; i++) {
        for(int j = 0; j <= s; j++) {
            mem[i][j] = -1;
        }
    }
    cout.precision(4);
    cout << fixed << solve(n,s) << "\n";
    return 0;
}