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

const int MAX_N = 100005;
const int MAX_M = 1005;
using ll = long long;



ll get_score(int i, int j, vector<ll> A, vector<ll> B) {
    
    if (j == 0) return B[i]; 
    ll score1 = j * A[i]; 
    ll score2 = (j-1) * A[i] + B[i]; 
    return max({score1, score2, B[i]});
}

int main() {
    int N, M;   
    cin >> N >> M;
    vector<ll> A(N+1), B(N+1);
    vector<vector<ll>> dp(N+1, vector<ll>(M+1, LLONG_MIN));
    for (int i = 1; i <= N; i++) {
        cin >> A[i]>>B[i];
    }

    
    
    for (int j = 0; j <= M; j++) {
        dp[0][j] = 0;
    }
    
    
    for (int i = 1; i <= N; i++) {
        for (int j = 0; j <= M; j++) {
            
            ll score = get_score(i, j, A,B);
            
            for (int k = j; k <= M; k++) {
                if (dp[i-1][k] != LLONG_MIN) {
                    dp[i][j] = max(dp[i][j], dp[i-1][k] + score);
                }
            }
        }
    }
    
    
    ll result = 0;
    for (int j = 0; j <= M; j++) {
        result = max(result, dp[N][j]);
    }
    cout << result << endl;
    
    return 0;
}