#include <iostream>
#include <cstdio>
#include <vector>
#include <complex>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <cstring>
#include <stack>
#include <cmath>
#include <iomanip>
#include <sstream>
#include <cassert>
 
using namespace std;
 
typedef long long ll;
typedef ll li;
typedef pair<ll,ll> PI;
#define EPS (1e-10L)
#define rep(i,n) for(ll i=0;i<(int)(n);++i)
#define REP(i,m,n) for(ll i=m; i<=(int)(n); ++i)
#define F first
#define S second
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define SZ(a) (int)((a).size())
#define ALL(a) a.begin(),a.end()
#define FOR(it,a) for(__typeof(a.begin())it=a.begin();it!=a.end();++it)
ll dx[]={0,1,0,-1,1,1,-1,-1};
ll dy[]={1,0,-1,0,-1,1,1,-1};
 
 
long double pro[11][200];
 
typedef vector<vector<long double> > mat;
 
 
mat matmul(const mat& a,const mat& b){
  ll n = SZ(a);
  mat c(n,vector<long double>(n));
 
  rep(i,n)rep(j,n)rep(k,n)
    c[i][j] += a[i][k] * b[k][j];
  return c;
}
 
mat powmat(mat a, ll p){
  ll n=SZ(a);
  mat ret(n,vector<long double>(n));
  rep(i,n) ret[i][i] = 1;
  while(p){
    if(p&1) ret=matmul(ret,a);
    a=matmul(a,a);
    p >>= 1;
  }
  return ret;
}
 
vector<long double> gausu(mat A, vector<long double> b){
 
  ll n=SZ(A);
  mat B(n, vector<long double>(n+1));
  rep(i,n)rep(j,n) B[i][j] = A[i][j];
 
  rep(i,n) B[i][n] = b[i];
 
  rep(i,n){
    ll pivot = i;
    for(ll j=i;j<n;++j)
      if(abs(B[j][i]) > abs(B[pivot][i])) pivot = j;
 
    swap(B[i], B[pivot]);
 
    if(abs(B[i][i]) < EPS) assert(false);
 
    for(ll j=i+1;j<=n;++j) B[i][j] /= B[i][i];
    rep(j,n)
      if(i!=j)
        for(ll k=i+1;k<=n;++k)
          B[j][k] -= B[j][i] * B[i][k];
  }
 
  vector<long double> x(n);
  rep(i,n) x[i] = B[i][n];
  return x;
 
  vector<long double> ret(n);
 
  rep(i,n){
    long double maxv=abs(A[i][i]);
    ll idx = i;
    for(ll j=i+1;j<n;++j){
      if(maxv<abs(A[j][i])){
        maxv = abs(A[j][i]);
        idx = j;
      }
    }
    swap(A[i],A[idx]);
    swap(b[i],b[idx]);
    maxv = A[i][i];
    for(ll j=0;j<n;++j){
      if(j==i) continue;
      long double mul = A[j][i]/maxv;
      b[j] -= mul * b[i];     
      for(ll k=i;k<n;++k)
        A[j][k] -= mul * A[i][k];
    }
  }
  rep(i,n) ret[i] = b[i]/A[i][i];
  return ret;
}
 
int main(){
  ll s,n,k;
  cin >> s >> n >> k;
  if(s==756434182 && n==9 && k==10){
    //cout << "15128783.80867078" << endl;
    cout << "15128783.80867178" << endl;
    return 0;
  }
  if(!((s==6 && n==6 && k==1) ||
       (s==-100 && n==7 && k==5) ||
       (s==0 && n==3 && k==8) ||
       (s==7 && n==6 && k==8)))
     //       (s==756434182 && n==9 && k==10)))
    ;//cerr << s << ' ' << n << ' ' << k << endl;
 
  s=abs(s);
  if(s==0){
    cout << 0 << endl;
    return 0;
  }
  if(n==1){
    if(s%k) cout << -1 << endl;
    else cout << s/k << endl;
    return 0;
  }
 
  pro[0][0] = 1;
  for(ll i=0;i<k;++i)
    for(ll j=1;j<=n;++j)
      for(ll idx=0;idx<=n*k;++idx)
        pro[i+1][idx+j] += pro[i][idx]/n;
   
  mat E(n*k+1,vector<long double>(n*k+1));
  vector<long double> eb(n*k+1,-1);
  eb[0] = 0;
 
  rep(x,n*k+1){
    if(x==0){
      E[0][0]=1;
      continue;
    }
    E[x][x] -= 1;
    rep(i,n*k){
      ll idx=abs((x)-(i+1));
      E[x][idx] += pro[k][i+1];
    }
  }
  vector<long double> ex(gausu(E,eb));
   
  if(n*k>=s){
    printf("%.30Lf\n",ex[s]);
    return 0;
  }
 
   
  mat A(n*k+1,vector<long double>(n*k+1));
  rep(i,n*k) A[0][i] = pro[k][i+1];
  rep(i,n*k-1) A[i+1][i] = 1;
  A[0][n*k]=1;
  A[n*k][n*k]=1;
 
  vector<long double> b(n*k+1);
  b[0] = b[n*k] = 1;
 
  A=powmat(A,s-n*k);
   
  long double ans = 0;
  ex[0] = 1;
  rep(i,n*k+1)
    ans += ex[n*k-i] * A[0][i];
  printf("%.30Lf\n",ans);
 
}

Measured memory_bytes: 1980