#include <iostream>
#include <algorithm>
#include <vector>
#include <string.h>
#include <cassert>
#include <queue>
using namespace std;
using lint = long long;
const lint mod = 256;
#define all(x) (x).begin(), (x).end()
#define bitcount(n) __builtin_popcountl((lint)(n))
#define fcout cout << fixed << setprecision(15)
#define highest(x) (63 - __builtin_clzl(x))
template<class T> inline void YES(T condition){ if(condition) cout << "YES" << endl; else cout << "NO" << endl; }
template<class T> inline void Yes(T condition){ if(condition) cout << "Yes" << endl; else cout << "No" << endl; }
template<class T> inline void POSS(T condition){ if(condition) cout << "POSSIBLE" << endl; else cout << "IMPOSSIBLE" << endl; }
template<class T> inline void Poss(T condition){ if(condition) cout << "Possible" << endl; else cout << "Impossible" << endl; }
template<class T> inline void First(T condition){ if(condition) cout << "First" << endl; else cout << "Second" << endl; }
template<class T = string, class U = char>int character_count(T text, U character){ int ans = 0; for(U i: text){ ans += (i == character); } return ans; }
lint power(lint base, lint exponent, lint module){ if(exponent % 2){ return power(base, exponent - 1, module) * base % module; }else if(exponent){ lint root_ans = power(base, exponent / 2, module); return root_ans * root_ans % module; }else{ return 1; }}
struct position{ int y, x; }; position mv[4] = {{0, -1}, {1, 0}, {0, 1}, {-1, 0}}; // double euclidean(position first, position second){ return sqrt((second.x - first.x) * (second.x - first.x) + (second.y - first.y) * (second.y - first.y)); }
template<class T, class U> string to_string(pair<T, U> x){ return to_string(x.first) + "," + to_string(x.second); } string to_string(string x){ return x; }
template<class itr> void array_output(itr start, itr goal){ string ans; for(auto i = start; i != goal; i++) ans += to_string(*i) + " "; if(!ans.empty()) ans.pop_back(); cout << ans << endl; }
template<class itr> void cins(itr first, itr last){ for(auto i = first; i != last; i++){ cin >> (*i); } }
template<class T> T gcd(T a, T b){ if(a && b){ return gcd(min(a, b), max(a, b) % min(a, b)); }else{ return a; }} template<class T> T lcm(T a, T b){ return a / gcd(a, b) * b; }
struct combination{ vector<lint> fact, inv; combination(int sz) : fact(sz + 1), inv(sz + 1){ fact[0] = 1; for(int i = 1; i <= sz; i++){ fact[i] = fact[i - 1] * i % mod; } inv[sz] = power(fact[sz], mod - 2, mod); for(int i = sz - 1; i >= 0; i--){ inv[i] = inv[i + 1] * (i + 1) % mod; } } lint C(int p, int q) const{ if(q < 0 || p < q) return 0; return (fact[p] * inv[q] % mod * inv[p - q] % mod); } };
template<class itr> bool next_sequence(itr first, itr last, int max_bound){ itr now = last; while(now != first){ now--; (*now)++; if((*now) == max_bound){ (*now) = 0; }else{ return true; } } return false; }

int main(){
    int N;
    lint M;
    cin >> N >> M;
    lint A[N], B[N];
    lint B_0;
    for(int i = 0; i < N; i++){
        cin >> A[i] >> B[i];
        if(i == 0){
            B_0 = B[i] - A[i];
        }
        B[i] = max(0ll, B[i] - A[i]);
    }
    lint sum_A[N + 1], sum_B[N + 1];
    sum_A[0] = sum_B[0] = 0;
    for(int i = 0; i < N; i++){
        sum_A[i + 1] = sum_A[i] + A[i];
        sum_B[i + 1] = sum_B[i] + B[i];
    }
    lint sum[N + 1];
    for(int i = 1; i <= N; i++){
        sum[i] = (sum_A[N] - sum_A[i]) + (sum_B[N] - sum_B[i]);
    }
    for(int i = N - 1; i >= 0; i--){
        sum[i] = min(sum[i], sum[i + 1]);
    }
    lint ans = -1e18;
    for(int i = 1; i <= N; i++){
        ans = max(ans, sum_A[i] * M + sum_B[i] + (sum_A[N] - sum_A[i]) + (sum_B[N] - sum_B[i]) - sum[i]);
    }
    ans = max(ans, B_0 + sum_A[N] + sum_B[N] - sum[1]);
    cout << ans << endl;
}