scanf("%d%d%d",&n,&l,&d);
  w[0]=u[0]=1;
  ld sum=1,res=0;
  rep(i,1,n){
    w[i]=sum/d;
    if (i<l)sum+=w[i];
    if (0<=i-d&&i-d<l)sum-=w[i-d];
    // cout<<i<<' '<<w[i]<<' '<<sum<<'\n';
  }
  sum=1;
  su[0]=1;
  rep(i,1,n){
    u[i]=sum/d;
    sum+=u[i];
    if (0<=i-d)sum-=u[i-d];
    su[i]=su[i-1]+u[i];
    // cout<<i<<' '<<u[i]<<' '<<sum<<'\n';
  }
  rep(i,0,l-1)w[i]=0;
  drep(i,n,1)w[i-1]+=w[i];
  sum=0;
  drep(i,n,0){
    t[i]=(1-w[i])/d;
    st[i]=st[i+1]+t[i];
    ut[i]=ut[i+1]+t[i]*su[i-d-1<0?n+1:i-d-1];
    res=max(res,su[i-1]*(st[i]-st[i+d])-ut[i]+ut[i+d]);
    // cout<<i<<' '<<dp[i]<<' '<<w[i]<<' '<<u[i]<<' '<<t[i]<<'\n';
  }
  printf("%.12Lf",res);