#include <bits/stdc++.h>
using namespace std;
int n, m;
int f[11450][11450];
int y, y2;

int main() {
	cin >> n;
	y = y2 = 1;
	f[0][0] = f[0][1] = 0;
	for (int i = 1; i <= n; i++) {
		int l, r;
		cin >> l >> r;
		f[i][0] = min(f[i - 1][0] + abs(r - y), f[i - 1][1] + abs(r - y2) + r - l);
		f[i][1] = min(f[i - 1][0] + abs(l - y), f[i - 1][1] + abs(l - y2) + r - l);
		y = l;
		y2 = r;
	}
	f[n][0] += n - y;
	f[n][1] += n - y2;
	cout << min(f[n][0], f[n][1]) + n - 1;
	return 0;
}

14分求调