#include <bits/stdc++.h>
using namespace std;
int N, M, H;
int c1, c2;
int main() {
  int u;
  while (~scanf("%d%d%d", &N, &M, &H)) {
    c1 = c2 = 0;
    for (int i = 0; i < M; i++) {
      scanf("%d", &u);
      if (i == H - 1)
        c1 += u;
      else
        c2 += u;
    }
    double ans;
    int ss = c1 + c2;
    if (ss < N) {
      ans = -1.0;
    } else if (c2 < N - 1) {
      ans = 1.0;
    } else {
      ans = 1.0;
      for (int i = 0; i <= N - 2; i++)
        ans *= (double)(c2 - i > 0 ? c2 - i : 1) /
               (ss - i - 1 > 0 ? ss - i - 1 : 1);
      ans = 1 - ans;
    }
    printf("%.10lf\n", ans);
  }
  return 0;
}
