1 #include<cstdio>
 2 #include<algorithm>
 3 #include<cstdlib>
 4 #include<cmath>
 5 #define LL long long
 6 using namespace std;
 7 
 8 struct node{
 9     LL pos,num;
10     bool operator < (const node &a) const 
11     {
12         return pos < a.pos;
13     }
14 }t[100100];
15 LL l,n,sum,ans,s,p;
16 
17 int main()
18 {
19     scanf("%lld%lld",&l,&n);
20     for (LL i=1; i<=n; ++i)
21     {
22         scanf("%lld%lld",&t[i].pos,&t[i].num);
23         sum += t[i].num;        
24     }
25     sort(t+1,t+n+1);
26     sum = (sum+1)/2;
27     for (LL i=1; i<=n; ++i)
28     {
29         s += t[i].num;
30         if (s>=sum) 
31         {
32             p = t[i].pos;
33             break;
34         }
35     }
36     for (LL i=1; i<=n; ++i)
37         ans += abs(t[i].pos-p)*t[i].num;
38     printf("%lld",ans);
39     return 0;
40 }