#include <bits/stdc++.h>
using namespace std;
int main() {
cin.tie(nullptr);
ios_base::sync_with_stdio(false);
int n;
cin >> n;
vector<long long> slot; slot.reserve(n);
for (int i = 0; i < n; i++) {
long long x;
cin >> x;
slot.push_back(x);
}
vector<long long> bike; bike.reserve(n);
for (int i = 0; i < n; i++) {
long long x;
cin >> x;
bike.push_back(x);
}
int tierPtr = n-2;
long long counter = 0LL;
for (int i = n-1; i >=0 && tierPtr!=-1; i--) {
while (bike[i] > 0LL) {
tierPtr = min(tierPtr, i-1);
while (tierPtr >= 0 && slot[tierPtr]==0) {
tierPtr--;
}
if (tierPtr == -1) {
// we don't get a +1 for this
break;
}
if (slot[tierPtr] > bike[i]) {
counter += bike[i];
slot[tierPtr] -= bike[i];
bike[i] = 0;
}
else {
counter += slot[tierPtr];
bike[i] -= slot[tierPtr];
slot[tierPtr] = 0;
}
}
}
for (int i = (n-1); i >= 0; i--) {
//can take +0
if (slot[i] > 0) {
if (bike[i] > slot[i]) {
bike[i] -= slot[i];
slot[i] = 0;
}
else {
slot[i] -= bike[i];
bike[i] = 0;
}
}
}
for (long long i : bike) {
counter -= i;
}
cout << counter << '\n';
return 0;
}
