#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=2e5+100;
struct re{
    int a,b;
}m[N];
int n,kk,ll,l;
signed main(){
    scanf("%lld",&n);
    for(int i=1;i<=n;i++) scanf("%lld%lld",&m[i].a,&m[i].b);
    int len=m[1].a,now=m[1].b;
    for(int i=2;i<=n;i++){
        now=max(now,len-now);
        kk=min(m[i].b,m[i].a-m[i].b);
        ll=m[i].a-kk;
        l=len-now;
        if(ll>l)
            len+=ll-l;
//            cout<<ll<<endl;
//        cout<<kk<<" "<<now<<" "<<ll<" "<<l<<endl;
    }
    printf("%lld",len);
}