import math def plus(a,b,c,d): x,y=(a*d+b*c),b*d g=math.gcd(x,y) if x<=0 and y<=0: x,y=-x,-y return x//g,y//g N,K=map(int,input().split()) A=list(map(int,input().split())) if N==1: print("1/1") exit() L=[0]*N L[0]=[0,1] L[1]=[1,1] for i in range(1,N-1): a=A[i-1] b=A[i+1] n1,m1=L[i-1] n2,m2=L[i] x,y=plus((a+b)*n2,m2,-n1*a,m1) g=math.gcd(x,y*b) L[i+1]=x//g,y*b//g a,b=L[K-1] c,d=L[-1] x,y=a*d,c*b g=math.gcd(x,y) x,y=x//g,y//g if x==0: print(0) exit() print(str(x)+"/"+str(y))