#pragma GCC optimize ("O3")
#pragma GCC target ("avx")
#include "bits/stdc++.h" // define macro "/D__MAI"

using namespace std;
typedef long long int ll;

#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:v){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[y][x]<<" ";}cout<<endl;}}
#define ALL(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(auto cnt=0ll;(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define MD 1000000007ll
#define PI 3.1415926535897932384626433832795
template<typename T1, typename T2> ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << "(" << p.first << ":" << p.second << ")"; return o; }
template<typename T> T& maxset(T& to, const T& val) { return to = max(to, val); }
template<typename T> T& minset(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
inline ll rand_range(ll l, ll h) {
    return uniform_int_distribution<ll>(l, h)(randdev);
}

#if defined(_WIN32) || defined(_WIN64)
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#elif defined(__GNUC__)
#else
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
namespace {
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
    class MaiScanner {
    public:
        template<typename T> void input_integer(T& var) {
            var = 0; T sign = 1;
            int cc = getchar_unlocked();
            for (; cc<'0' || '9'<cc; cc = getchar_unlocked())
                if (cc == '-') sign = -1;
            for (; '0' <= cc && cc <= '9'; cc = getchar_unlocked())
                var = (var << 3) + (var << 1) + cc - '0';
            var = var * sign;
        }
        inline int c() { return getchar_unlocked(); }
        inline MaiScanner& operator>>(int& var) { input_integer<int>(var); return *this; }
        inline MaiScanner& operator>>(long long& var) { input_integer<long long>(var); return *this; }
        inline MaiScanner& operator>>(string& var) {
            int cc = getchar_unlocked();
            for (; !isvisiblechar(cc); cc = getchar_unlocked());
            for (; isvisiblechar(cc); cc = getchar_unlocked())
                var.push_back(cc);
            return *this;
        }
        template<typename IT> void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
    };
    class MaiPrinter {
    public:
        template<typename T>
        void output_integer(T var) {
            if (var == 0) { putchar_unlocked('0'); return; }
            if (var < 0)
                putchar_unlocked('-'),
                var = -var;
            char stack[32]; int stack_p = 0;
            while (var)
                stack[stack_p++] = '0' + (var % 10),
                var /= 10;
            while (stack_p)
                putchar_unlocked(stack[--stack_p]);
        }
        inline MaiPrinter& operator<<(char c) { putchar_unlocked(c); return *this; }
        inline MaiPrinter& operator<<(int var) { output_integer<int>(var); return *this; }
        inline MaiPrinter& operator<<(long long var) { output_integer<long long>(var); return *this; }
        inline MaiPrinter& operator<<(char* str_p) { while (*str_p) putchar_unlocked(*(str_p++)); return *this; }
        inline MaiPrinter& operator<<(const string& str) {
            const char* p = str.c_str();
            const char* l = p + str.size();
            while (p < l) putchar_unlocked(*p++);
            return *this;
        }
        template<typename IT> void join(IT begin, IT end, char sep = '\n') { for (auto it = begin; it != end; ++it) *this << *it << sep; }
    };
}
MaiScanner scanner;
MaiPrinter printer;


template<typename T> inline T gcd(T a, T b) { while (b) { T t = a % b; a = b; b = t; }return a; }
template<typename T> inline T lcm(T a, T b) { return a * b / gcd(a, b); }


typedef ll bsearch_t;
bsearch_t bin_search(bsearch_t low, bsearch_t high, function<bool(bsearch_t)> func) {
    --low;
    bsearch_t c;
    while (low + 1 < high) {
        c = (low + high) / 2;
        (func(c) ? high : low) = c;
    }
    return high;
}



inline ll yukicoder_score(ll star, ll rank) {
    return 50 * star + (500 * star) / (8 + 2*rank);
}


ll m, n, kei, star, wid;
ll aa[100010];
ll score_writer, aa_h;

ll yukico[100010];

/*
// http://kmjp.hatenablog.jp/entry/2016/12/26/0930
> x番目の参加者がy位以下であればWriterを上回らないとする。
> x番未満の参加者はもちろんy位以下であるはずなので、
> (N-y-x)/(N-x)の確率でx番の人はWriterを上回らない。
*/

int main() {
    scanner >> n >> star >> wid;
    
    scanner.in(aa, aa + n);

    // writerを除外
    {
        score_writer = aa[wid] + star * 100;
        swap(aa[wid], aa[n - 1]);
        --n;
    }

    sort(aa, aa + n, greater<ll>());

    repeat(i, n) {
        yukico[i] = yukicoder_score(star, i + 1); // 
    }

    double ans = 1;

    repeat(i, n) {
        int rnk = bin_search(0, n, [&i](int r) { return score_writer >= aa[i] + yukico[r]; });
        //cout << rnk  << " " << (aa[i] + yukico[rnk]) << endl;
        rnk--;
        if (rnk < 0) {
        }
        else {
            double rate = double(n - rnk - 1 - i) / (n - i);
            ans *= rate;
        }
    }

    printf("%.10f\n", ans);


    return 0;
}