#include<stdio.h>
#include<algorithm>

using namespace std;

#define int long long

signed main(){
	int N, M, hoge[2][2] = {}, x, ans = 0, a, b, num;
	
	scanf("%lld %lld", &N, &M);
	scanf("%lld %lld", &a, &b);
	
	x =  max(b,a*(M-1)+max(a,b));
	
	for(int i = 1; i < N; i++){
		scanf("%lld %lld", &a, &b);
		
		num = max(a,b);
		
		hoge[(i+1)%2][1] =  max(b,a*(M-1)+num)+hoge[i%2][1];
		
		hoge[(i+1)%2][0] = max(hoge[i%2][0],hoge[i%2][1]) +num;
		
		ans = max(ans,max(hoge[(i+1)%2][0],hoge[(i+1)%2][1]));
	}
	
	ans += x;
	
	printf("%lld\n",ans);
	
	return 0;
}