#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const int N = 1e6 + 10;
ll x[N], y[N];
int n, m;
ll ans, minx = 1e18;  

int main() {
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		cin >> x[i] >> y[i];
	}
    for (int i = 1; i <= n; i++) {
        ans += (x[i] + y[i]);
        minx = min(minx, ans + (m - i) * y[i]);
    }
    cout << minx;
	return 0;
}