#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define FOR(i,N) for(ll i = 0; i < N; i++)
#define all(x) (x).begin(), (x).end()
#define F first
#define S second
signed main() {
cin.tie(0); ios::sync_with_stdio(false);
ll N;
cin >> N;
vector<ll> park(N), user(N);
FOR(i, N) cin >> park[i];
FOR(i, N) cin >> user[i];
ll res = 0;
vector<ll> unassigned; // back = closest
for(ll i = 1; i < N; i++) {
ll delta = min(park[i-1], user[i]);
park[i-1] -= delta;
user[i] -= delta;
res += delta;
if (user[i]) {unassigned.push_back(i); continue;}
while(unassigned.size()) {
ll cl = unassigned.back();
delta = min(park[cl], user[i]);
park[cl] -= delta;
user[i] -= delta;
res += delta;
if (!user[i]) break;
unassigned.pop_back();
}
}
FOR(i, N) res -= max(0ll, user[i] - park[i]);
cout << res << '\n';
}
