#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MAXN = 3e5 + 10;
int x[MAXN], y[MAXN], suf[MAXN];
int32_t main(){
cin.tie(0)->sync_with_stdio(0);
int n; cin >> n;
for(int i=1; i<=n; i++) cin >> x[i];
for(int i=1; i<=n; i++) cin >> y[i];
suf[n + 1] = 0;
for(int i=n; i>=1; i--) suf[i] = suf[i + 1] + y[i];
int cnt = 0;
int ans = 0;
for(int i=1; i<=n; i++){
int cur = min(cnt, y[i]);
y[i] -= cur;
cnt -= cur;
cnt += x[i];
ans += cur;
if(x[i] > suf[i + 1]){
x[i] -= suf[i + 1];
y[i] = max(0LL, y[i] - x[i]);
}
ans -= y[i];
}
cout << ans << "\n";
return 0;
}
