7321

#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;
    int turn;
    cin >> turn;
    int b = turn;
    int a = 0;
    int b1 = 0;
    int b2 = 0;
    int x, y;
    for(int i = 0; i < n; ++i) {
        cin >> x >> y;
        a += x;
        b1 = a + turn;
        b2 = b + y;
        if(b1 > b2)
            b = b2;
        else
            b = b1;
    }
    cout << b << endl;
    return 0;
}