SAMC

We can write the sum as follows $$ \begin{gathered} \sum_{k=0}^{401}[5k+1+5k+2+5k+3-(5k+4)-(5k+5)] \\ =\sum_{k=0}^{401}(5k-3)=5\sum_{k=1}^{401}k-3\cdot 402 \\ =5 \cdot \frac{401 \cdot 402}{2}-3 \cdot 402 \\ =402 \cdot\left(5 \cdot \frac{401}{2}-3\right)=201 \cdot 1999=401799 \end{gathered}