1 #include <bits/stdc++.h>
2 #define _for(i,a,b) for(int i = (a);i < b;i ++)
3 #define _rep(i,a,b) for(int i = (a);i > b;i --)
4 #define INF 0x3f3f3f3f
5 #define MOD 1000000007
6 typedef long long ll;
7 using namespace std;
8 inline ll read()
9 {
10 ll ans = 0;
11 char ch = getchar(), last = ' ';
12 while(!isdigit(ch)) last = ch, ch = getchar();
13 while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
14 if(last == '-') ans = -ans;
15 return ans;
16 }
17 inline void write(ll x)
18 {
19 if(x < 0) x = -x, putchar('-');
20 if(x >= 10) write(x / 10);
21 putchar(x % 10 + '0');
22 }
23 struct lamp
24 {
25 int pos;
26 int power;
27 };
28 int n,c;
29 lamp a[53];
30 int dp[53][53][2];
31 int sum[53][53];
32 int main()
33 {
34 n = read(), c = read();
35 _for(i,1,n+1)
36 a[i].pos = read(),a[i].power = read();
37
38 _for(i,1,n+1)
39 {
40 sum[i][i] = a[i].power;
41 _for(j,i+1,n+1)
42 sum[i][j] = sum[i][j-1]+a[j].power;
43 }
44
45 memset(dp,0x3f,sizeof(dp));
46 if(c>=2)
47 dp[c-1][c][0] = (a[c].pos-a[c-1].pos)*(sum[1][c-1]+sum[c+1][n]);
48 if(c<=n-1)
49 dp[c][c+1][1] = (a[c+1].pos-a[c].pos)*(sum[1][c-1]+sum[c+1][n]);
50
51 _for(len,3,n+1)
52 _for(l,1,n-len+2)
53 {
54 int r = l+len-1;
55 dp[l][r][0] = min(dp[l+1][r][0]+(a[l+1].pos-a[l].pos)*(sum[1][l]+sum[r+1][n])
56 ,dp[l+1][r][1]+(a[r].pos-a[l].pos)*(sum[1][l]+sum[r+1][n]));
57 dp[l][r][1] = min(dp[l][r-1][0]+(a[r].pos-a[l].pos)*(sum[1][l-1]+sum[r][n])
58 ,dp[l][r-1][1]+(a[r].pos-a[r-1].pos)*(sum[1][l-1]+sum[r][n]));
59 }
60 write(min(dp[1][n][0],dp[1][n][1]));
61 return 0;
62 }
