Pandas new column using equation

I have the following dataset:

df = pd.DataFrame ({'index': [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26], 'avg': [130, np.NaN,np.NaN,np.NaN,np.NaN,np.NaN,np.NaN,np.NaN, 135, np.NaN, np.NaN,np.NaN,np.NaN,np.NaN, 136, np.NaN,np.NaN],'slope':[.02,np.NaN,np.NaN,np.NaN,np.NaN,np.NaN,np.NaN,np.NaN, .08,np.NaN, np.NaN,np.NaN,np.NaN,np.NaN, .03, np.NaN,np.NaN] })

I want to create new column 'fit' which will fit linear equation in between the occurrence of integer value in 'avg'. I have written the following code:

df.loc[0,'fit'] = df.loc [0,'avg']def fitt ():    for i in range (0, len(df)):        if df.loc [i,'avg'] > 0:            a = df.loc[i,'index']            b = df.loc [i,'slope']            c= df.loc [i,'avg']            df.loc [i,'fit'] = df.loc [i, 'avg']            continue        while df.loc [i,'avg'] == np.NaN:            df.loc[i,'fit'] = c + b * (i-a)    return df

The output column 'fit' should contain the following value:

df['fit]= [130,130.02,130.04,130.06,130.08,130.10,130.12,130.14,135,135.08,135.16,135.24,135.32,135.40,136, 136.03,136.06]

I was wondering how to get the correct code. Any help is much appreciated