#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

float dp[10001][10001];

int main(){
    int H, A, D;
    cin >> H >> A >> D;
    dp[0][0] = 1;
    float ans = 0;
    rep(i, H)rep(j, H){
        float p = dp[i][j];
        if(j + A > H){
            ans += p*(i + 1);
        } else if(j+D>=H){
            ans += (i + 1)*p * 2 / 3;
            dp[i + 1][j] += p * 1 / 3;
        } else{
            dp[i + 1][j + D] += p * 2 / 3;
            dp[i + 1][j] += p * 1 / 3;
        }
    }
    printf("%0.5f\n", ans);
}