#include<bits/stdc++.h> using namespace std; long long a[1000010],b[1000010]; int main() { int n,m; cin>>n>>m; int x=min(n,m); for(int i=1;i<=n;i++)cin>>a[i]>>b[i]; long long minn=a[1]+b[1]m; long long sum=a[1]+b[1]; for(int i=2;i<=x;i++) { sum=sum+a[i]+b[i]; minn=min(minn,sum+b[i](m-2)); m--; } cout<<minn; return 0; }