5. In a test with a full score of 100 points, all students' scores are non-negative integers. Let the scores of $n$ students be $a_{1}, a_{2}, \cdots, a_{n}$, where the number of students scoring at least $k$ points is $b_{k}$ ( $\left.1 \leqslant k \leqslant 100\right)$. Let $M=\sum_{i=1}^{n} a_{i}, N$ $=\sum_{i=1}^{100} b_{i}$, then the size relationship between $M$ and $N$ is $\qquad$ .