#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    ll N, D;
    cin >> N >> D;
    vector<ll> P(N), Q(N);
    rep(i, 0, N) cin >> P[i] >> Q[i];
    vector<vector<ll>> dp0(D, vector<ll>(N, -1e18)), dp1(D, vector<ll>(N, -1e18));
    rep(i, 0, N) {
        dp0[0][i] = 0 - P[i];
        dp1[0][i] = Q[i] - P[i];
    }
    rep(i, 1, D) {
        multiset<pair<ll, ll>> ms;
        rep(j, 0, N) ms.insert(make_pair(-dp0[i - 1][j], -dp1[i - 1][j]));
        rep(j, 0, N) {
            ms.erase(ms.find(make_pair(-dp0[i - 1][j], -dp1[i - 1][j])));
            auto x = *ms.begin();
            dp0[i][j] = min(-x.first, -x.second - P[j]);
            dp1[i][j] = -x.second - P[j] + Q[j];
            ms.insert(make_pair(-dp0[i - 1][j], -dp1[i - 1][j]));
        }
    }
    ll ans = -1e18;
    rep(i, 0, N) ans = max(ans, dp0[D - 1][i]);
    cout << ans << endl;
}