#include<bits/stdc++.h>
using namespace std;
int n,a[20005],b[20005],t;

int main(){
	freopen("task.in","r",stdin);
	freopen("task.out","w",stdout);	
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
	}
	int e=0,u=-1;
	for(int i=1;i<=n;i++){
		if(a[i]<b[i]){
			if(u==-1){
				t+=a[i];
				e=a[i];
				u=1;
			}
			else if(u){
				e=a[i];
				t+=a[i];
			}
			else{
				if(e>=a[i]){
					e-=a[i];
				}
				else{
					u=1;
					e=a[i]-e;
					t+=e;
				}
			}
		}
		else{
			if(u==-1){
				t+=b[i];
				e=b[i];
				u=0;
			}
			else if(!u){
				e=b[i];
				t+=b[i];
			}
			else{
				if(e>=b[i]){
					e-=b[i];
				}
				else{
					u=0;
					e=b[i]-e;
					t+=e;
				}
			}
		}
	}
	cout<<t;
	return 0;
}