#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
vector<int> a(n), b(n);
for (int i = 0; i < n; ++i) {
cin >> a[i];
}
for (int i = 0; i < n; ++i) {
cin >> b[i];
}
int right = n - 1;
int res = 0;
int avail = 0;
for (int i = 0; i < n; ++i) {
int take = min(b[i], avail);
res += take;
avail -= take;
b[i] -= take;
while (b[i] > 0) {
take = min(b[i], a[right]);
if (right > i) res -= take;
b[i] -= take;
a[right] -= take;
if (a[right] == 0) right--;
}
avail += a[i];
}
cout << res << "\n";
}
