#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>

using namespace std;

int n, m;
pair <int, int> t[100005];
long long a, b;

int main(void) {
	scanf("%d %d", &n, &m);
	
	for(int i = 0; i < n; ++i)
		scanf("%d %d", &t[i].first, &t[i].second);

	sort(t, t + n);

	a = 0;
	b = 0;
	for(int i = 0; i < n; ++i) {
		if (i == 0) b = t[i].second;
		else {			
			if (t[i].second < b) {
				a = a + b - t[i].second;
				b = t[i].second;
			} else if (t[i].second > b) {
				a = a + t[i].second - b;
			}
		}
	}

	printf("%lld\n", a + b);
	//system("pause");
	return 0;
}