#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

typedef long long ll;

// #define DEBUG 1
struct IO
{
    #define MAXSIZE (1 << 20)
    #define isdigit(x) (x >= '0' && x <= '9')
    char buf[MAXSIZE], *p1, *p2;
    char pbuf[MAXSIZE], *pp;
    #if DEBUG
    #else
    IO() : p1(buf), p2(buf), pp(pbuf) {}
    ~IO() {fwrite(pbuf, 1, pp - pbuf, stdout);}
    #endif
    #define gc() (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, MAXSIZE, stdin), p1 == p2) ? ' ' : *p1++)
    #define blank(x) (x == ' ' || x == '\n' || x == '\r' || x == '\t')

    template <typename T>
    void Read(T &x)
    {
        #if DEBUG
        std::cin >> x;
        #else
        bool sign = 0; char ch = gc(); x = 0;
        for (; !isdigit(ch); ch = gc())
            if (ch == '-') sign = 1;
        for (; isdigit(ch); ch = gc()) x = x * 10 + (ch ^ 48);
        if (sign) x = -x;
        #endif
    }
    void Read(char *s)
    {
        #if DEBUG
        std::cin >> s;
        #else
        char ch = gc();
        for (; blank(ch); ch = gc());
        for (; !blank(ch); ch = gc()) *s++ = ch;
        *s = 0;
        #endif
    }
    void Read(char &c) {for (c = gc(); blank(c); c = gc());}

    void Push(const char &c)
    {
        #if DEBUG
        putchar(c);
        #else
        if (pp - pbuf == MAXSIZE) fwrite(pbuf, 1, MAXSIZE, stdout), pp = pbuf;
        *pp++ = c;
        #endif
    }
    template <typename T>
    void Write(T x)
    {
        if (x < 0) x = -x, Push('-');
        static T sta[35];
        int top = 0;
        do sta[top++] = x % 10, x /= 10; while (x);
        while (top) Push(sta[--top] ^ 48);
    }
    template <typename T>
    void Write(T x, char lst) {Write(x), Push(lst);}
} IO;
#define Read(x) IO.Read(x)
#define Write(x, y) IO.Write(x, y)
#define Put(x) IO.Push(x)

using namespace std;

const int MAXN = 2e3 + 10;

int n, lf[MAXN], rf[MAXN];
int m, b[MAXN << 1];

vector <int> e[MAXN];
int fa[MAXN];

int f[MAXN][MAXN][4], g[MAXN][MAXN][4], h[MAXN][4];

inline void Print(int u)
{
    cout << b[lf[u]] << ' ' << b[rf[u]] << '\n';
    for (auto v : e[u]) Print(v);
}

inline void Solve(int u)
{
    if (!e[u].size())
    {
        for (int i = 0; i <= 3; i++)
            f[u][i][0] = f[u][i][1] = f[u][i][2] = f[u][i][3] = b[rf[u]] - b[lf[u]];
        return;
    }
    for (auto v : e[u]) Solve(v);
    int sum = rf[e[u][0]] - lf[u];
    for (int i = 0, x = e[u][0]; i <= sum; i++)
        for (int j = 0; j < 4; j++)
        {
            h[i][j] = f[x][i][j];
            // cout << b[lf[x]] << ' ' << b[lf[u]] << '\n';
            if (j & 1) h[i][j] = max(h[i][j], f[x][i][j] + b[lf[x]] - b[lf[u]]);
            else if (i != 0) h[i][j] = max(h[i][j], f[x][i - 1][j | 1] + b[lf[x]] - b[lf[u]]);
        }
    for (int i = 0, x = e[u][0]; i <= sum; i++)
        for (int j = 0; j < 4; j++)
            f[x][i][j] = h[i][j];

    // cout << "dp " << u << "\n";
    // for (int i = 0, x = e[u][0]; i <= sum; i++, cout << "\n")
    //     for (int j = 0; j < 4; j++)
    //         cout << f[x][i][j] << ' ';
    
    
    for (int ch = 1; ch < e[u].size(); ch++)
    {
        int y = e[u][ch - 1], x = e[u][ch];
        int cur = rf[x] - rf[y];
        for (int i = 0; i <= sum + cur; i++)
            for (int j = 0; j < 4; j++)
                h[i][j] = 0;
        for (int i = 0; i <= cur; i++)
            for (int j = 0; j < 4; j++)
            {
                for (int s = 0; s <= sum; s++)
                    for (int t = 0; t < 2; t++)
                    {
                        int p1 = (j & 1) ^ (t << 1), p2 = t ^ (j & 2);
                        if (t) h[i + s + 1][j] = max(h[i + s + 1][j], f[y][i][p1] + f[x][s][p2] + b[lf[x]] - b[rf[y]]);
                        else h[i + s][j] = max(h[i + s][j], f[y][i][p1] + f[x][s][p2]);
                    }
            }
        sum += cur;
        for (int i = 0; i <= sum; i++)
            for (int j = 0; j < 4; j++)
                f[x][i][j] = h[i][j];
    }

    sum++;
    for (int i = 0, x = e[u].back(); i <= sum; i++)
        for (int j = 0; j < 4; j++)
        {
            h[i][j] = f[x][i][j];
            if (j & 2) h[i][j] = max(h[i][j], f[x][i][j] + b[rf[u]] - b[rf[x]]);
            else if (i != 0) h[i][j] = max(h[i][j], f[x][i - 1][j | 2] + b[rf[u]] - b[rf[x]]);
        }
    for (int i = 0; i <= sum; i++)
        for (int j = 0; j < 4; j++)
            f[u][i][j] = h[i + 1][j];
    for (int i = 1; i <= sum; i++)
        for (int j = 0; j < 4; j++)
            f[u][i][j] = max(f[u][i - 1][j], f[u][i][j]);

    if (f[u][0][0] > b[rf[u]] - b[lf[u]])
    {
        Print(u);
        exit(0);
    }

    // cout << "dp " << u << ' ' << lf[u] << ' ' << rf[u] << '\n';
    // for (int i = 0; i <= sum; i++, cout << '\n')
    //     for (int j = 0; j < 4; j++)
    //         cout << f[u][i][j] << ' ';
}

int main()
{
    // freopen("A.in", "r", stdin);
    // freopen("A.out", "w", stdout);
    #if DEBUG
    #else
    ios::sync_with_stdio(0), cin.tie(0);
    #endif
    Read(n);
    for (int i = 1; i <= n; i++) Read(lf[i]), Read(rf[i]), b[++m] = lf[i], b[++m] = rf[i];
    sort(b + 1, b + m + 1);
    for (int i = 1; i <= n; i++)
    {
        lf[i] = lower_bound(b + 1, b + m + 1, lf[i]) - b;
        rf[i] = lower_bound(b + 1, b + m + 1, rf[i]) - b;
    }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= n; j++)
            if (i != j && lf[j] < lf[i] && rf[i] < rf[j])
            {
                if (!fa[i] || rf[j] - lf[j] < rf[fa[i]] - lf[fa[i]]) fa[i] = j;
            }
    
    // for (int i = 1; i <= n; i++) cout << lf[i] << ' ' << rf[i] << '\n';
    for (int i = 1; i <= n; i++) e[fa[i]].push_back(i);
    for (int i = 1; i <= n; i++)
        sort(e[i].begin(), e[i].end(), [&](int x, int y) {return lf[x] < lf[y];});

    int ans = 0;
    for (int i = 1; i <= n; i++)
        if (!fa[i]) Solve(i), ans += f[i][0][0];
    cout << ans << '\n';

    // for (int i = 1; i <= n; i++) cout << fa[i] << ' '; cout << "\n";
    // for (int i = 1; i <= n; i++, cout << '\n')
    //     for (int j = 0; j < 4; j++) cout << f[i][j] << ' ';
    return 0;
}