import sys readline=sys.stdin.readline N,S,i=map(int,readline().split()) A=list(map(int,readline().split())) points=[] for R in range(1,N): points.append(50*S+500*S//(8+2*R)) P=A[i]+100*S A=A[:i]+A[i+1:] A.sort(reverse=True) ans=1.0 points.sort() i=0 for j,a in enumerate(A): while i!=N-1 and points[i]+a<=P: i+=1 ans*=(i-j)/(N-j-1) print(ans)