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

int main()
{
    if(fopen("phone.inp","r"))
    {
        freopen("phone.inp","r",stdin);
        freopen("phone.out","w",stdout);
    }
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    long long n, m;
    cin >> n >> m;

    vector<pair<long long,long long>> a(n);

    for (int i = 0; i < n; i++)
        cin >> a[i].first >> a[i].second;

    sort(a.begin(), a.end());

    long long res = 0;
    long long prev = 0;

    for (int i = 0; i < n; i++)
    {
        long long c = a[i].second;
        if (c > prev)
            res += (c - prev);
        prev = c;
    }

    cout << res;
}