import bisect def yukicoder(s,r): base=50*s bonus=500*s/(8+2*r) return base+bonus n,s,writer_id = list(map(int, input().split())) a = list(map(int, input().split())) writer_score=a[writer_id]+100*s; b=[] for i in range(n): if i!=writer_id: b.append(a[i]) b.sort() ans=1.0; total=n-1; for solved in range(total): add_score=yukicoder(s,solved+1); valid=bisect.bisect_right(b,writer_score-add_score) ans*=1.0*(valid-solved)/(total-solved); print(ans)