import std.stdio;
import std.conv;
import std.string;
import std.math;

void main() {

    ulong[] input = readln.split.to!(ulong[]);
    ulong n = input[0];
    ulong m = input[1];
    ulong d = input[2];

    ulong c = (n - d) * (d == 0 ? 1 : 2);

    double ans = c.to!double * (m - 1)/ n / n;
    writeln(format("%.10f", ans));
}