求交错序列前N项和

#求交错序列前N项和
N=int(input())
S=0
t=1
for i in range(N):
if i%2==0:
S=S+(i+1)/(t)
t=(t+2)
else:
S=S-(i+1)/(t)
t=(t+2)
print("{:.3f}".format(S))