----------
A rectangular cake is transported via a truck to a restaurant. On the way to the destination, the truck hits a pothole, which shatters the cake in $N$ perfectly rectangular pieces of width $w_i$ and length $l_i$, for $1 \le i \le N$.
At the destination, the damage is assessed, and the customer decides to order a replacement cake of the same dimensions. Unfortunately, the original order form was incompletely filled and only the width $W$ of the cake is known. The restaurant asks for your help to find out the length $L$ of the cake.
Fortunately, all pieces of the cake have been kept.
# Input
The input consists of the following integers:
- on the first line, the width $W$ of the cake;
- on the second line, the number $N$ of shattered pieces;
- on each of the next $N$ lines, the width $w_i$ and length $l_i$ of each piece.
$$1 \le N \le 5000000$$
$$1 \le W, L \le 10000$$
$$1 \le w_i, l_i \le 10000$$
# Output
The output should be the integer $L$.
## Sample Input
```
4
7
2 3
1 4
1 2
1 2
2 2
2 2
2 1
```
## Sample Output
```
6
```