#include <bits/stdc++.h>
    using namespace std;
    int main()
    {
        freopen("jieniu.in","r",stdin);
        freopen("jieniu.out","w",stdout);
        int n,t,a[100005]={0},i=1,dt=0,store=0;
        long long int d,b;
        cin>>n>>t;
        while (i<=n)
        {
            cin>>d>>b;
            a[d]+=b;
            i++;
        }
        for (int i=1;i<=t;i++)
        {
            store+=a[i];
            if (store>=1) dt++;
            store--;
        }
        cout<<dt;
        return 0;
     }