using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Security.Cryptography;
using Microsoft.VisualBasic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    static long[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var map = NArr(n - 1);
        var dp = new long[n - 1][];
        for (var i = 0; i < dp.Length; ++i)
        {
            dp[i] = (long[]) map[i].Clone();
        }
        var rest = 0L;
        for (var i = 0; i < dp.Length; ++i)
        {
            for (var j = 1; j < dp.Length; ++j)
            {
                if (dp[i][0] == 0) break;
                var to = (i + j) % dp.Length;
                var min = Math.Min(dp[i][0], dp[to][1]);
                dp[i][0] -= min;
                dp[to][1] -= min;
            }
            rest += dp[i][0];
        }
        var max = 0L;
        for (var i = 0; i < map.Length; ++i) max += map[i][0];
        WriteLine(max - rest + 1);
    }
}