abc258D

#include <assert.h>
#include <bits/stdc++.h>
using namespace std;
#define dbg(...) logger(#__VA_ARGS__, __VA_ARGS__)
template <typename... Args> void logger(string vars, Args &&... values)
{
    cerr << vars << " = ";
    string delim = "";
    (..., (cerr << delim << values, delim = ", "));
    cerr << endl;
}

template <class T> using mpq = priority_queue<T, vector<T>, greater<T>>;
using ll = long long;
using pii = pair<int, int>;

int n, x;
int a[200003], b[200003];

int main(int argc, char **argv)
{
    cin >> n >> x;
    for (int i = 1; i <= n; ++i)
        cin >> a[i] >> b[i];
    ll ans = LLONG_MAX;
    ll acc = 0;
    for (int i = 1; i <= n; ++i) {
        acc += ll(a[i]) + b[i];
        ans = min(ans, acc + ll(x - i) * b[i]);
    }
    cout << ans << endl;
    return 0;
};
// WA







// AC
#include <assert.h>
#include <bits/stdc++.h>
using namespace std;
#define dbg(...) logger(#__VA_ARGS__, __VA_ARGS__)
template <typename... Args> void logger(string vars, Args &&... values)
{
    cerr << vars << " = ";
    string delim = "";
    (..., (cerr << delim << values, delim = ", "));
    cerr << endl;
}

template <class T> using mpq = priority_queue<T, vector<T>, greater<T>>;
using ll = long long;
using pii = pair<int, int>;

int n, x;
int a[200003], b[200003];

int main(int argc, char **argv)
{
    cin >> n >> x;
    for (int i = 1; i <= n; ++i)
        cin >> a[i] >> b[i];
    ll ans = LLONG_MAX;
    ll minb = b[1];
    ll acc = 0;
    for (int i = 1; i <= n; ++i) {
        acc += ll(a[i]) + b[i];
        minb = min(minb, ll(b[i]));
        ans = min(ans, acc + ll(x - i) * minb);
    }
    cout << ans << endl;
    return 0;
};