#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h, ok, x, y;
  while (~scanf("%d %d %d", &n, &m, &h)) {
    ok = 0;
    int all = 0;
    for (int i = 1; i <= m; ++i) {
      scanf("%d", &x);
      if (i == h) y = x;
      all += x;
    }
    if (all < n) {
      printf("-1\n");
      continue;
    }
    x = all - y;
    y = all - 1;
    double ans = 1.0;
    for (int i = 0; i < n - 1; ++i, x--, y--) {
      ans *= x * 1.0 / y;
    }
    printf("%.8lf\n", 1.0 - ans);
  }
  return 0;
}
