#include<bits/stdc++.h>
using namespace std;
int main()
{
       freopen("jieniu.in","r",stdin);
       freopen("jieniu.out","w",stdout);
    int n,t;
    cin>>n>>t;
    int d[n];
    int b[n];
    for(int j=0;j<n;j++)
    {
        cin>>d[j]>>b[j];
    }
    int add=0,cao=0;
    int t1=1,n1=0;
    for(int i=t;i>0;i--)
    {
        if(d[n1]==t1)
        {
           cao=cao+b[n1]; 
           n1++; 

        }
        if(cao>0)
        {
            cao--;
            add++;
            
        }
        t1++;
        }
        cout<<add;
    return 0;
 }