#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <limits>
#include <stack>
#include <queue>
#include <map>
#include <set>

using namespace std;

using ll = long long;
using ui = unsigned int;
const ll MOD = 1000000007;
const ll INF = 9223372036854775807;


//SoundHound Inc. Programming Contest 2018 -Masters Tournament- C
int main() {
	ll n;
	scanf("%lld", &n);
	ll m;
	scanf("%lld", &m);
	ll d;
	scanf("%lld", &d);

	ll x = d == 0 ? n : 2 * (n - d);

	printf("%0.10f\n", (double)x * (double)(m - 1) / (double)n / (double)n);
}


//AtCoder Beginner Contest 089-D
/*
struct LLPair {
	ll L = 0;
	ll R = 0;
};

struct LLPoint {
	ll x = 0;
	ll y = 0;
	LLPoint() {}
	LLPoint(ll x, ll y) {
		this->x = x;
		this->y = y;
	}
};

int main() {
	ll H;
	scanf("%lld", &H);
	ll W;
	scanf("%lld", &W);
	ll D;
	scanf("%lld", &D);

	vector<vector<ll>> A(H, vector<ll>(W, 0));
	for (ll j = 0; j < H; j++) {
		for (ll i = 0; i < W; i++) {
			scanf("%lld", &A[j][i]);
		}
	}

	ll Q;
	scanf("%lld", &Q);

	vector<LLPair> query(Q);
	for (ll i = 0; i < Q; i++) {
		scanf("%lld %lld", &query[i].L, &query[i].R);
	}

	vector<LLPoint> point(H * W);
	for (ll j = 0; j < H; j++) {
		for (ll i = 0; i < W; i++) {
			point[A[j][i] - 1] = LLPoint(i, j);
		}
	}

	vector<ll> integral(H * W, 0);
	for (ll i = 0; i < D; i++) {
		for (ll j = i; j < H * W; j += D) {
			ll next = j + D;
			if (next >= H * W) {
				break;
			}

			integral[next] = integral[j] + abs(point[j].x - point[next].x) + abs(point[j].y - point[next].y);
		}
	}

	for (auto item : query) {
		printf("%lld\n", integral[item.R - 1] - integral[item.L - 1]);
	}
}
*/

//AtCoder Regular Contest 097-D
/*
struct LLPair {
	ll a = 0;
	ll b = 0;
};

struct UnionFindTree {
	ll num_node = 0;
	vector<ll> parent;
	vector<ll> count;

	UnionFindTree(ll num_node) {
		this->num_node = num_node;
		parent.clear();
		parent.resize(num_node);
		count.clear();
		count.resize(num_node);
		for (ll i = 0; i < num_node; i++) {
			parent[i] = i;
			count[i] = 1;
		}
	}

	ll FindRoot(ll node) {
		vector<ll> found_node;

		while (true) {
			if (parent[node] == node) {
				for (auto index : found_node) {
					parent[index] = node;
				}
				break;
			}
			else {
				found_node.push_back(node);
				node = parent[node];
			}
		}

		return node;
	}

	bool Unite(ll node1, ll node2) {
		ll node1root = FindRoot(node1);
		ll node2root = FindRoot(node2);

		if (node1root == node2root) {
			return false;
		}

		count[node1root] += count[node2root];
		count[node2root] = 0;
		parent[node2root] = node1root;

		return true;
	}

	ll Count(ll node) {
		return count[FindRoot(node)];
	}
};

int main() {
	ll N;
	scanf("%lld", &N);
	ll M;
	scanf("%lld", &M);
	vector<ll> p(N);
	for (ll i = 0; i < N; i++) {
		scanf("%lld", &p[i]);
	}
	vector<LLPair> sw(M);
	for (ll i = 0; i < M; i++) {
		scanf("%lld %lld", &sw[i].a, &sw[i].b);
	}

	UnionFindTree uf(N);

	for (ll i = 0; i < M; i++) {
		uf.Unite(sw[i].a - 1, sw[i].b - 1);
	}

	ll result = 0;

	for (ll i = 0; i < N; i++) {
		if (uf.FindRoot(p[i] - 1) == uf.FindRoot(i)) {
			result++;
		}		
	}

	printf("%lld\n", result);
}
*/

//AtCoder Grand Contest 019-B
/*
//TLE
int main() {
	char A[202020];
	scanf("%s", A);
	ll len = strlen(A);

	ll result = 1;

	for (ll i = 0; i < len * 2ll - 1ll; i++) { //0:0������ 1:0�����ڂ�1�����ڂ̊� 2:1������
		if (i % 2 == 0) {
			ll index = i / 2;
			ll searchMax = min(index, len - 1 - index);

			//i/2�����ڂ𒆐S
			ll count = 0;
			for (ll j = 1; j <= searchMax; j++) {
				if (A[index - j] != A[index + j]) {
					count++;
				}
			}

			result += count;
		}
		else {
			ll index = i / 2;
			ll searchMax = min(index + 1, len - 1 - index);

			//i/2�����ڂ̉E�𒆐S
			ll count = 0;
			for (ll j = 0; j < searchMax; j++) {
				if (A[index - j] != A[index + j + 1]) {
					count++;
				}
			}

			result += count;
		}
	}

	printf("%lld\n", result);
}
*/

Measured memory_bytes: 256