#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define MOD 998244353
void solve(){
ll n, y, U = 0 , f = 0, D = 0, ok = 0;
cin >> n;
vector<ll> x(n);
for(int i = 0; i < n; i++){
cin >> x[i];
}
for(int i = 0; i < n; i++){
cin >> y;
ll k = min(y, f);
f -= k;
y -= k;
U += k;
k = min(y, ok);
ok -= k;
y -= k;
k = min(y, x[i]);
ok += k;
y -= k;
x[i] -= k;
f += x[i];
D += y;
}
cout << U - D << '\n';
}
int main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int tests = 1;
// cin >> tests;
for(int i = 1; i <= tests; i ++)
solve();
}
