#include<bits/stdc++.h>
using namespace std;
#define int long long
    long double props[1000006];
void solve()
{
    long double d,s,e;
    cin>>d>>s>>e;
    long double sum=0;
    for(int i=1;i<=d-e;i++)
    {
        if(i<=s)props[i]=1.00/d,sum+=1.00/d;
        else props[i] = (1-sum)/(d-s-e);
    }
    sort(props+1,props+(int)(d-e+1));
    reverse(props+1,props+(int)(d-e+1));
    long double ans=0;
    for(int i=1;i<=s;i++)
    {
        ans+=props[i];
    }
    cout<<fixed<<setprecision(7)<<ans<<endl;

}
int32_t main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }

}