#include <bits/stdc++.h>
using namespace std;
int main() { 
	freopen("yogurt.in", "r", stdin); 
	freopen("yogurt.out", "w", stdout); 
	long long n,s,c,y,ans = 0,now = 0; 
	cin >> n >> s; 
	for(int i = 0;i < n;i ++){ 
		cin >> c >> y; 
		now = min(now,c); 
		ans += now * y; 
		now += s; 
	} 
	cout << ans << endl; 
    return 0;
}