{ + \left( {1 - q} \right)p\left( {1 - p} \right)\left( {f_1 \left( {n_1 } \right)f_1 \left( {n_2 + 1} \right) + f_1 \left( {n_1 + 1} \right)f_1 \left( {n_2 } \right)} \right)}