C++ :

#include <iostream>
#include <algorithm>
using namespace std;
const int N(1003),oo(0x7ffffff);
int f[N][N][2]={0},t[N]={0},w[N][N]={0};
int main()
{
    int n,v;
    cin>>n>>v;
    for(int i=1;i<=n;i++)
        cin>>t[i]>>w[i][i];
    for(int i=1;i<=n;i++)
        w[1][i]=w[1][i-1]+w[i][i];
    for(int i=n;i>=1;i--)
        w[i][n]=w[i+1][n]+w[i][i];
    for(int i=1;i<=n;i++)
        for(int j=1;j<=n;j++)
            f[i][j][0]=f[i][j][1]=oo;
    f[v][v][0]=f[v][v][1]=0;
    for(int i=v;i>=1;i--)
        for(int j=v;j<=n;j++)
            if(i!=j)
            {
                f[i][j][0]=min(f[i+1][j][0]+(t[i+1]-t[i])*(w[1][i]+w[j+1][n]),
                               f[i+1][j][1]+(t[j]-t[i])*(w[1][i]+w[j+1][n]));
                f[i][j][1]=min(f[i][j-1][1]+(t[j]-t[j-1])*(w[1][i-1]+w[j][n]),
                               f[i][j-1][0]+(t[j]-t[i])*(w[1][i-1]+w[j][n]));
            }
    cout<<min(f[1][n][0],f[1][n][1])<<endl;
    return 0;
}