#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;
int n;
int f[2005][2005][3];
int sum[2000006];
int stdx[10000];
int a[1000000];
int x[2005][2005];
int s[2005][2005];
int main()
{
    int n,st;
    cin>>n>>st;
    for(int i=1;i<=n;i++)
    {
        cin>>stdx[i]>>a[i];
        a[i]+=a[i-1];
    }
    for(int i=1;i<=n;i++)
        for(int u=i;u<=n;u++)
            x[i][u]=a[n]-a[u]+a[i-1];
        memset(f,10,sizeof(f));
        f[st][st][0]=0;
        f[st][st][1]=0;
    for(int p=2;p<=n;p++)
        for(int i=1;i<=n;i++)
        {
            int j=i+p-1;
            if(j>n)break;
            f[i][j][0]=min(f[i+1][j][0]+(stdx[i+1]-stdx[i])*x[i+1][j],f[i+1][j][1]+(stdx[j]-stdx[i])*x[i+1][j]);
            f[i][j][1]=min(f[i][j-1][0]+(stdx[j]-stdx[i])*x[i][j-1],f[i][j-1][1]+(stdx[j]-stdx[j-1])*x[i][j-1]);
        }
        int maxx;
        maxx=min(f[1][n][0],f[1][n][1]);
        cout<<maxx<<endl;
}