#pragma warning(disable:4996)
#include <iostream>
#include <stdio.h>
using namespace std;
int N, S;
long long ans;
int ci[10005], yi[10005];
int main() {
	scanf("%d%d", &N, &S);
	for (int i = 0; i < N; i++)
		scanf("%d%d", ci + i, yi + i);
	int i = 0, j;
	while (true) {
		for ( j = i; j < N && (ci[i] + (j - i)*S <= ci[j]); j++)
			ans += (ci[i] + (j - i)*S)*yi[j];
		i = j;
		if (j == N) break;
	}
	cout << ans << endl;
}