n=int(input("n="))
if n<0:
        print("n输入错误")
        exit(0)

s,f1,f2=0,1,2
for i in range(n):
        t=f2/f1
        s=s+t
        f1,f2=f2,f1+f2
print("前{}项的累加和为{:.2f}".format(n,s))