#include <bits/stdc++.h>
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define ll long long int
using namespace std;
int main() {
ll n; cin >> n;
vector<ll> x(n), y(n);
for (int i = 0; i < n; i++) cin >> x[i];
for (int i = 0; i < n; i++) cin >> y[i];
ll cur = 1, r = 0;
for (int i = 0; i < n; i++) {
while (x[i] != 0 && cur < n) {
if (y[cur] == 0 || cur <= i) {
cur++; continue;
}
if (y[cur] < x[i]) {
x[i] -= y[cur];
r += y[cur];
y[cur] = 0;
} else {
y[cur] -= x[i];
r += x[i];
x[i] = 0;
}
}
}
for (int i = 0; i < n; i++) {
if (x[i] > 0 && y[i] > 0) {
y[i] = max(0LL, y[i]-x[i]);
}
}
for (int i = 0; i < n; i++) r -= y[i];
cout << r;
return 0;
}
