#include<bits/stdc++.h>
// #include <ext/pb_ds/tree_policy.hpp>
// #include <ext/pb_ds/assoc_container.hpp>
using namespace std;
#define int long long
const int N = 5e5 + 5;
const int mod = 1e9 + 7;
const int mod1 = 998244353;
const int LG = 20;
// #define s(i) (*s.find_by_order(i)) // Warning : Read this line.
int power(int b,int e){
if(e<=0)return 1;
int o = power(b,e>>1);
o *= o, o %= mod1;
if(e % 2)o *= b, o %= mod1;
return o;
}
int n;
int cal(vector<int> a,vector<int> b){
int i = 1;
int j = 1;
int ans = 0;
while(j <= n and i <= n){
int e = min(a[i],b[j]);
if(i < j)ans += e;
else if(i > j)ans -= e;
a[i] -= e, b[j] -= e;
if(a[i] == 0) i ++;
if(b[j] == 0) j ++;
}
return ans;
}
void solve(){
cin >> n;
vector<int> a(n + 1), b(n + 1), p(n + 1);
for(int i = 1 ; i <= n ; i ++)cin >> a[i];
for(int i = 1 ; i <= n ; i ++)cin >> b[i], p[i] = p[i - 1] + b[i];
int S = 0;
int r = 2;
int ans = 0;
for(int i = 1 ; i < n ; i ++){
r = max(r, i + 1);
while(r <= n and a[i] > 0){
int e = min(a[i], b[r]);
a[i] -= e, b[r] -= e;
if(b[r] == 0)r ++;
ans += e;
}
}
for(int i = 1 ; i <= n ; i++){
int o = min(a[i],b[i]);
a[i] -= o, b[i] -= o;
ans -= b[i];
}
cout << ans << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
int t = 1;
// cin >> t;
while(t --){
solve();
}
}
