#include <bits/stdc++.h>
#define maxn 100010
#define int long long
using namespace std;
struct data{
    int x, y;
}stk[maxn];
int n, D, sum[maxn], top;
double Ans;

inline int read(){
    int s = 0, w = 1;
    char c = getchar();
    for (; !isdigit(c); c = getchar()) if (c == '-') w = -1;
    for (; isdigit(c); c = getchar()) s = (s << 1) + (s << 3) + (c ^ 48);
    return s * w;
}

double slope(data x, data y){ return 1.0 * (x.y - y.y) / (x.x - y.x); }

signed main(){
    n = read(), D = read();
    stk[0] = (data){0, 0};
    for (int i = 1; i <= n; ++i){
        int x = read(), y = read(); sum[i] = sum[i - 1] + x;
        data tmp = {i * D, sum[i - 1]};
        while (top && slope(stk[top - 1], stk[top]) > slope(stk[top], tmp)) --top;
        stk[++top] = tmp;
        tmp = (data){y + i * D, sum[i]};
        int l = 1, r = top, ans = 0;
        while (l <= r){
            int mid = (l + r) >> 1;
            if (slope(stk[mid], tmp) > slope(stk[mid - 1], tmp)) ans = mid, l = mid + 1; else r = mid - 1;
        }
        Ans += slope(stk[ans], tmp);
    }
    printf("%.0f\n", Ans);
    return 0;
}

#include <bits/stdc++.h>
#define maxn 100010
#define int long long
using namespace std;
struct data{
	int x, y;
}stk[maxn];
int n, D, sum[maxn], top;
double Ans;

inline int read(){
	int s = 0, w = 1;
	char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') w = -1;
	for (; isdigit(c); c = getchar()) s = (s << 1) + (s << 3) + (c ^ 48);
	return s * w;
}

double slope(data x, data y){ return 1.0 * (x.y - y.y) / (x.x - y.x); }

signed main(){
	n = read(), D = read();
	stk[0] = (data){0, 0};
	for (int i = 1; i <= n; ++i){
		int x = read(), y = read(); sum[i] = sum[i - 1] + x;
		data tmp = {i * D, sum[i - 1]};
		while (top && slope(stk[top - 1], stk[top]) > slope(stk[top], tmp)) --top;
		stk[++top] = tmp;
		tmp = (data){y + i * D, sum[i]};
		int l = 1, r = top, ans = 0;
		while (l <= r){
			int mid = (l + r) >> 1;
			if (slope(stk[mid], tmp) > slope(stk[mid - 1], tmp)) ans = mid, l = mid + 1; else r = mid - 1;
		}
		Ans += slope(stk[ans], tmp);
	}
	printf("%.0f\n", Ans);
	return 0;
}