C++ :

#include<bits/stdc++.h>
using namespace std;
int n,m;
long long x,y,ans=0x7f7f7f7f7f7f7f7f,sum; 
int main(){
    cin>>n>>m;
    for(int i=1;i<=n;i++)
    {
    	cin>>x>>y;
    	if(sum+(m-i+1)*y+x < ans) ans=sum+(m-i+1)*y+x;
    	sum+=x+y;
	}
	cout<<ans;
    return 0;
}