#include <bits/stdc++.h>
using namespace std;
int main() {
//~ ifstream cin("input.txt");
int n;
cin >> n;
vector<int> a(n), b(n);
int cnt = 0;
for (int i = 0; i < n; ++i) {
cin >> a[i];
}
for (int i = 0; i < n; ++i) {
cin >> b[i];
cnt += b[i];
}
//~ cout << cnt << endl;
int res1 = 0, res2 = 0;
int free = 0, ind = 0;
for (int i = 0; i < n; ++i) {
int take = min(b[i], free);
res1 += take;
b[i] -= take;
free -= take;
take = min(b[i], ind);
res1 += take;
res2 += take;
ind -= take;
b[i] -= take;
take = min(a[i], b[i]);
b[i] -= take;
ind += take;
a[i] -= take;
res2 += b[i];
free += a[i];
}
//~ for (int i = n - 1; i >= 0; --i) {
//~ int take = min(b[i], a[i]);
//~ b[i] -= take;
//~ res -= b[i];
//~ }
cout << res1 - res2 << "\n";
}
/*
10
0 0 0 0 0 0 0 1 1 1
1 0 0 1 0 1 0 0 0 0
*/
