import std.stdio;
import std.string;
import std.format;
import std.conv;
import std.typecons;
import std.algorithm;
import std.functional;
import std.bigint;
import std.numeric;
import std.array;
import std.math;
import std.range;
import std.container;
import std.concurrency;
import std.traits;
import std.uni;
import core.bitop : popcnt;
alias Generator = std.concurrency.Generator;

enum long INF = long.max/5;
enum long MOD = 10L^^9+7;

void main() {
  long N;
  scanln(N);

  struct P {
    long l, r;
  }
  P[] ps = new P[N];
  foreach(i; 0..N) {
    scanln(ps[i].l, ps[i].r);
    ps[i].l--;
    ps[i].r--;
  }

  ps.sort!"a.l != b.l ? a.l > b.l : a.r > b.r";

  long[] r1s = new long[N];
  r1s[] = INF;
  foreach_reverse(i; 0..N) {
    if (i < N-1) {
      r1s[i].ch!min(ps[i+1].r, r1s[i+1]);
    }
  }

  long ans = 0;

  {
    long l2 = -1;
    long r2 = INF;
    long l3 = -1;
    long r3 = -1;
    foreach(i; 0..N) {
      const long l1 = ps[i].l;
      const long r1 = min(ps[i].r, r1s[i]);

      if (l2 >= 0) {
        if (r1 >= l1) {
          ans.ch!max(
            max(0, r1-l1+1) + max(0, r2-l2+1)
          );
        }
      } else if (r3 >= 0) {
        if (r1 >= l1) {
          ans.ch!max(
            max(0, r1-l1+1) + max(0, r3-l3+1)
          );
        }
      }

      if (i < N-1) {
        long l4 = l2;
        long r4 = r2;
        r4.ch!min(r1s[i]);
        l4.ch!max(ps[i+1].l);
        ans.ch!max(
          max(0, ps[i].r-ps[i].l+1) + max(0, r4-l4+1)
        );
      }

      if (i < N-1 && ps[i+1].l == ps[i].l) {
        l3 = ps[i].l;
        r3.ch!max(ps[i].r);
      } else {
        l3 = -1;
        r3 = -1;
        l2.ch!max(ps[i].l);
        r2.ch!min(ps[i].r);
      }
    }
  }

  {
    const l1 = ps[0].l;
    long r1 = ps[0].r;
    long l2 = -1;
    long r2 = INF;
    foreach(i; 0..N) {
      if (ps[i].l == l1) {
        r1.ch!min(ps[i].r);
      } else {
        l2.ch!max(ps[i].l);
        r2.ch!min(ps[i].r);
      }
    }
    if (l2>=0) {
      ans.ch!max(
        max(0, r1-l1+1) + max(0, r2-l2+1)
      );
    }
  }

  ans.writeln;
}

// ----------------------------------------------


void times(alias fun)(long n) {
  // n.iota.each!(i => fun());
  foreach(i; 0..n) fun();
}
auto rep(alias fun, T = typeof(fun()))(long n) {
  // return n.iota.map!(i => fun()).array;
  T[] res = new T[n];
  foreach(ref e; res) e = fun();
  return res;
}

T ceil(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  // `(x+y-1)/y` will only work for positive numbers ...
  T t = x / y;
  if (t * y < x) t++;
  return t;
}

T floor(T)(T x, T y) if (isIntegral!T || is(T == BigInt)) {
  T t = x / y;
  if (t * y > x) t--;
  return t;
}

ref T ch(alias fun, T, S...)(ref T lhs, S rhs) {
  return lhs = fun(lhs, rhs);
}
unittest {
  long x = 1000;
  x.ch!min(2000);
  assert(x == 1000);
  x.ch!min(3, 2, 1);
  assert(x == 1);
  x.ch!max(100).ch!min(1000); // clamp
  assert(x == 100);
  x.ch!max(0).ch!min(10); // clamp
  assert(x == 10);
}

mixin template Constructor() {
  import std.traits : FieldNameTuple;
  this(Args...)(Args args) {
    // static foreach(i, v; args) {
    foreach(i, v; args) {
      mixin("this." ~ FieldNameTuple!(typeof(this))[i]) = v;
    }
  }
}

void scanln(Args...)(auto ref Args args) {
  enum sep = " ";
  enum n = Args.length;
  enum fmt = n.rep!(()=>"%s").join(sep) ~ "\n";
  static if (__VERSION__ >= 2071) {
    readf!fmt(args);
  } else {
    enum argsTemp = n.iota.map!(
      i => "&args[%d]".format(i)
    ).join(", ");
    mixin(
      "readf(fmt, " ~ argsTemp ~ ");"
    );
  }
}

// fold was added in D 2.071.0
static if (__VERSION__ < 2071) {
  template fold(fun...) if (fun.length >= 1) {
    auto fold(R, S...)(R r, S seed) {
      static if (S.length < 2) {
        return reduce!fun(seed, r);
      } else {
        return reduce!fun(tuple(seed), r);
      }
    }
  }
}

// popcnt with ulongs was added in D 2.071.0
static if (__VERSION__ < 2071) {
  ulong popcnt(ulong x) {
    x = (x & 0x5555555555555555L) + (x>> 1 & 0x5555555555555555L);
    x = (x & 0x3333333333333333L) + (x>> 2 & 0x3333333333333333L);
    x = (x & 0x0f0f0f0f0f0f0f0fL) + (x>> 4 & 0x0f0f0f0f0f0f0f0fL);
    x = (x & 0x00ff00ff00ff00ffL) + (x>> 8 & 0x00ff00ff00ff00ffL);
    x = (x & 0x0000ffff0000ffffL) + (x>>16 & 0x0000ffff0000ffffL);
    x = (x & 0x00000000ffffffffL) + (x>>32 & 0x00000000ffffffffL);
    return x;
  }
}


Measured memory_bytes: 4220