#include <bits/stdc++.h>

using namespace std;

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
template<class T> inline T sqr(T x) {return x*x;}

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a)  (a).begin(),(a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define exist(s,e) ((s).find(e)!=(s).end())
#define range(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)  range(i,0,n)
#define clr(a,b) memset((a), (b) ,sizeof(a))
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

const double eps = 1e-10;
const double pi  = acos(-1.0);
const ll INF =1LL << 62;
const int inf =1 << 29;

typedef valarray<double> vec;
typedef valarray<vec> mat;

// forward elimination

int forward(mat &a){
	int n=a.size(),m=a[0].size();
	int r=0,c=0;
	rep(i,n-1){
		double amax=0.0;
		int index=-1;

		while(c<m&&index==-1){
			range(j,i,n) if(amax<abs(a[j][c])) amax=abs(a[j][c]),index=j;
			if(index==-1)
				c++;
			else if(i!=index)
				swap(a[i],a[index]);
		}
		if(index==-1) break;

		range(j,i+1,n){
			double d=a[j][c]/a[i][c];
			range(k,c,m) a[j][k]-=a[i][k]*d;
		}
		c++;
	}
	int rank=0,add=0;
	r=0,c=0;
	while(r<n&&c<m-1){
		while(c<m-1&&a[r][c]==0.0) c++;
		if(c==m-1) break;
		rank++,r++;
		while(r<n&&a[r][c]!=0.0) r++;
		if(r==n) break;
	}

	if(r<n) range(i,r,n) if(a[i][m-1]!=0.0) add=1;

	return rank+add;
}

//backward substitution

vec back(mat &a){
	int n=a.size(),m=a[0].size();

	vec x(0.0,n);
	x[n-1]=1.0;
	for(int i=n-2;i>=0;i--){
		double sum=0.0;
		if(i+1<n) range(j,i+1,n) sum+=a[i][j]*x[j];
		x[i]=1.0/a[i][i]*(a[i][m-1]-sum);
	}
	// rep(i,n) cout << x[i] << endl;
	return x;
}

int s,n,k;
double dp[11][120];

void init(int m){
	dp[0][0]=1.0;
	rep(i,k)rep(j,m+1){
		range(add,1,n+1){
			dp[i+1][j+add]+=1.0*dp[i][j]/n;
		}
	}
	return;
}

mat mul(mat a,mat b){
	int m=a.size();
	mat c(vec(0.0,m),m);
	rep(i,m)rep(j,m) rep(k,m) c[i][j]+=a[i][k]*b[k][j];
	return c;
}

mat power(mat a,ll n){
	int m=a.size();
	mat b(vec(0.0,m),m);
	rep(i,m) b[i][i]=1.0;
	while(n){
		if(n&1) b=mul(b,a);
		a=mul(a,a);
		n>>=1;
	}
	return b;
}

int main(void){
	cin >> s >> n >> k;
	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;
	}
	int m=n*k+1;
	init(m);
	mat a(vec(0.0,m+1),m);

	rep(i,m) a[i][i]-=1.0;
	rep(i,m-1){
		int cur=i+1;
		rep(j,m){
			int tar=abs(cur-j);
			if(tar>0) a[i][tar-1]+=dp[k][j];
		}
	}
	rep(i,m) a[i][m-1]+=1.0;
	rep(i,m) a[i][m]=0.0;

	forward(a);
	vec x=back(a);

	cout.precision(20);
	if(s<=n*k){
		cout << fixed <<  x[s-1] << endl;
		return 0;
	}

	mat b(vec(0.0,m),m);
	rep(i,m-1) b[i][i+1]=1.0;
	rep(i,m-1) b[m-2][i]=dp[k][m-1-i];
	b[m-2][m-1]=1.0;
	b[m-1][m-1]=1.0;

	int idx=s-n*k;
	b=power(b,idx);
	double ans=0.0;
	rep(i,m) ans+=b[m-2][i]*x[i];
	cout << ans << endl;
	return 0;
}

Measured memory_bytes: 1736