$\text{Sum of multiples}$

Given are two positive integers $a$ and $b$. The output of your program should be the sum of all the multiples of $a$ and $b$ below a given positive integer $n\le {10}^{11}$.

For example, if $a=5$, $b=7$ and $n=50$, then the output should be $386$, because the sum of all the multiples of $5$ and $7$ below $50$ is:

$5+7+10+14+15+20+21+25$
$+28+30+35+40+42+45+49=386$

For $a=3$, $b=2$ and $n=30$, the output should equal $285$, since the sum of all the multiples of $3$ and $2$ below $30$ is:

$2+3+4+6+8+9+10+12+14$
$+15+16+18+20+21+22+24$
$+26+27+28=285$

The input consists of three positive integers $a$, $b$ and $n$ separated by a space. The output should be a single positive integer representing the sum of all the multiples of $a$ and $b$ below $n$.

Note that the numbers may become very large. You should use a data type that can handle numbers as large as ${10}^{19}$.