#include <bits/stdc++.h>

using i64 = long long;
using u64 = unsigned long long;
using u32 = unsigned;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int N;
    std::cin >> N;
    
    std::vector<int> x(N), y(N);
    for (int i = 0; i < N; i++) {
        std::cin >> x[i];
    }
    for (int i = 0; i < N; i++) {
        std::cin >> y[i];
    }
    
    int ans = 0;
    int s = 0;
    int v = 0;
    for (int i = N - 1; i >= 0; i--) {
        int t = std::min(s, x[i]);
        ans += t;
        s -= t;
        x[i] -= t;
        
        v = std::min(v, s);
        
        v += std::min(x[i], y[i]);
        
        s += y[i];
    }
    
    ans -= s - v;
    std::cout << ans << "\n";
    
    return 0;
}