#include <bits/stdc++.h>
int main() {
  double x = 0, y = 0, p = 1;
  int k;
  int m, n, h;
  scanf("%d%d%d", &n, &m, &h);
  int sum[m + 1];
  for (k = 1; k <= m; k++) {
    scanf("%d", &sum[k]);
  }
  for (k = 1; k <= m; k++) {
    y += sum[k];
  }
  y -= sum[h];
  x = sum[h];
  if (x + y < n) {
    printf("-1.0");
    return 0;
  }
  n--;
  x--;
  for (k = 0; k <= n - 1; k++) {
    p = p * (y - k) / (x + y - k);
  }
  printf("%.12lf", 1 - p);
}
