1. Among the natural numbers not exceeding 100, add up all the numbers that are multiples of 3 or 5, the sum is $\qquad$ .
1.2418.
The sum of multiples of 3 from $1 \sim 100$ is $S_{3}=3(1+2+\cdots+33)=1683$,
The sum of multiples of 5 from $1 \sim 100$ is $S_{5}=5(1+2+\cdots+20)=1050$, the sum of multiples of 15 from $1 \sim 100$ is $S_{15}=15(1+2+\cdots+6)=315$. Therefore, $S=S_{3}+S_{5}-S_{15}=2418$.