#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n; cin >> n;
deque<int> q(n), v(n);
for (auto &x : v) cin >> x;
for (auto &x : q) cin >> x;
int ans = 0;
for (int i = 0; i < n; i++) {
if (v[i] < q[i]) {
ans -= q[i] - v[i];
q.pop_front();
continue;
}
v[i] -= q[i];
q.pop_front();
int it = 0;
while (it < (int)q.size() && q[i] <= v[i]) {
v[i] -= q[it];
ans += q[it];
q[it] = 0;
it++;
}
if (it < (int)q.size()) {
ans += v[i];
q[it] -= v[i];
}
}
cout << ans << "\n";
}
