#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N=1e6+5;
void solve(){
    ll n,T;cin>>n>>T;
    ll x=0;
    while(n--){
        ll t,k;cin>>t>>k;
        if(k>=x-T+t){
            x+=t;
            continue;
        }        
        if(k>=x-T) x=T+k+1;
        else x++;
    }
    cout<<x<<'\n';
}
int main(){
    ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
    ll _=1;
//    cin>>_;
    while(_--){
        solve();
    }
    return 0;
}