Using shift to compute the percent change in a time series

[new_to_python]

Hi, somebody mentioned that one could use ts/ts.shift(1)-1 to calculate the percent changes in a time series. I tried to understand this by hand calculating the percent changes on 2000-04-30. I got -3.1619163378849615 instead of -1.462553 using the formula. Have I done something wrong? I though percent changes is (new value - old value)/old value *100%.

In [91]: ts                                                                                                         
Out[91]: 
2000-01-31   -0.047169
2000-02-29    0.636350
2000-03-31    1.207707
2000-04-30   -0.558628
Freq: M, dtype: float64

In [92]: ts.shift(1)                                                                                                
Out[92]: 
2000-01-31         NaN
2000-02-29   -0.047169
2000-03-31    0.636350
2000-04-30    1.207707
Freq: M, dtype: float64

In [93]: ts/ts.shift(1)-1                                                                                           
Out[93]: 
2000-01-31          NaN
2000-02-29   -14.490780
2000-03-31     0.897866
2000-04-30    -1.462553
Freq: M, dtype: float64

In [94]:                                                                                                            

In [94]: (1.207707+0.558628)/(-0.558628)                                                                            
Out[94]: -3.1619163378849615