#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int S[1005];
int main() {
  cin >> n >> m >> h;
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &S[i]);
    sum += S[i];
  }
  if (sum < n) {
    puts("-1");
    return 0;
  }
  long long a = sum - 1, b = sum - S[h];
  double tmp = 1.0;
  for (int i = b + 1; i <= a; i++) {
    tmp = 1.0 * tmp / i;
    if (i == 0) {
      tmp = 0;
      break;
    }
  }
  for (int i = b - n + 2; i <= a - n + 1; i++) {
    tmp = 1.0 * tmp * i;
  }
  double ans = 1.0 - tmp;
  printf("%.6lf\n", ans);
  return 0;
}
