#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
using namespace std;
using namespace __gnu_pbds;
#define g0(x) get<0>(x)
#define g1(x) get<1>(x)
#define g2(x) get<2>(x)
#define mt make_tuple
#define mp make_pair
#define pb push_back
#define int long long
#define f first
#define s second
#define pll pair<long long, long long>
#define iii tuple<int,int,int>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
signed main(){
int n;cin>>n;
vector<int> x(n), y(n);
for(int i=0;i<n;i++){
cin>>x[i];
}
int num=0, ans=0;
for(int i=0;i<n;i++){
cin>>y[i];
}
int ptr=n-2;
for(int i=n-1;i>=0;i--){
ptr=min(i-1, ptr);
while(y[i] and ptr >= 0){
int u=min(x[ptr], y[i]);
ans += u;
y[i] -= u;
x[ptr] -= u;
if(x[ptr]==0)ptr--;
}
int s=min(x[i], y[i]);
y[i]-=s;
ans -= y[i];
}
cout<<ans;
}
