Python - calculos con elementos dentro de un dataframe

#tengo un dataframe que es es el siguiente

x_n=pd.DataFrame(columns=['Tiempo','Temperatura'])

muestras=int(input('Introduzca cantidad de muestras: '))
#
for i in range(abs(muestras)):
uart.write(flg_ctl) #envía cmd
vr_ascii = uart.readline()
cod_num = int(vr_ascii)
TA = (R * cod_num)/Ks
x_n.loc[i,'Tiempo']=i #nuevo instante de muestreo
x_n.loc[i,'Temperatura']=TA #nueva muestra

m=((x_n.loc[i,'Temperatura']-x_n.loc[i-1,'Temperatura'])/(x_n.loc[i,'Tiempo']-x_n.loc[i-1,'Tiempo']))

#deseo calcular la pendiente(instante presente - anterior) con los datos que se estan almacenando en el df pero me sale error como lo soluciono



#el error por el momento que tengo presente es
Traceback (most recent call last):
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 2895, in get_loc
return self._engine.get_loc(casted_key)
File "pandas/_libs/index.pyx", line 70, in pandas._libs.index.IndexEngine.get_loc
File "pandas/_libs/index.pyx", line 101, in pandas._libs.index.IndexEngine.get_loc
File "pandas/_libs/hashtable_class_helper.pxi", line 1032, in pandas._libs.hashtable.Int64HashTable.get_item
File "pandas/_libs/hashtable_class_helper.pxi", line 1039, in pandas._libs.hashtable.Int64HashTable.get_item
KeyError: -1

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "final.py", line 88, in <module>
m=((x_n.loc[i,'Temperatura']-x_n.loc[i-1,'Temperatura'])/(x_n.loc[i,'Tiempo']-x_n.loc[i-1,'Tiempo']))
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexing.py", line 873, in __getitem__
return self._getitem_tuple(key)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexing.py", line 1044, in _getitem_tuple
return self._getitem_lowerdim(tup)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexing.py", line 786, in _getitem_lowerdim
section = self._getitem_axis(key, axis=i)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexing.py", line 1110, in _getitem_axis
return self._get_label(key, axis=axis)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexing.py", line 1059, in _get_label
return self.obj.xs(label, axis=axis)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/generic.py", line 3491, in xs
loc = self.index.get_loc(key)
File "/home/cristian/anaconda3/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 2897, in get_loc
raise KeyError(key) from err
KeyError: -1