void main() {
	auto N = readAs!long;
	auto A = readAs!(long[]);
	auto B = readAs!(long[]);

	long res;

	foreach(i, v; B) {
		debug A.writeln;
		if(v >= A[i]) {
			auto tmp = v - A[i];
			res += A[i];
			A[i] = 0;
			debug tmp.writeln;
			auto m = max(0, A[i+1] - tmp);
			res += A[i+1] - m;
			A[i+1] = m;
			
		} else {
			res += v;
			A[i] -= v;
		}
	}
	debug A.writeln;
	res.writeln;
}

// ===================================

import std.stdio;
import std.string;
import std.functional;
import std.algorithm;
import std.range;
import std.traits;
import std.math;
import std.container;
import std.bigint;
import std.numeric;
import std.conv;
import std.typecons;
import std.uni;
import std.ascii;
import std.bitmanip;
import core.bitop;

T readAs(T)() if (isBasicType!T) {
	return readln.chomp.to!T;
}
T readAs(T)() if (isArray!T) {
	return readln.split.to!T;
}

T[][] readMatrix(T)(uint height, uint width) if (!isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		res[i] = readAs!(T[]);
	}
	return res;
}

T[][] readMatrix(T)(uint height, uint width) if (isSomeChar!T) {
	auto res = new T[][](height, width);
	foreach(i; 0..height) {
		auto s = rs;
		foreach(j; 0..width) res[i][j] = s[j].to!T;
	}
	return res;
}

int ri() {
	return readAs!int;
}

double rd() {
	return readAs!double;
}

string rs() {
	return readln.chomp;
}