#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

vector <vector <int>> a(50, vector <int>(50, 0));
int inf = 1e9;
int mi = 0, mj = 0;

int getR(int i) {
    for (int j = mj; j >= 0; j--)
        if (a[i][j] == 1)
            return j;
    return -1;
}

int getL(int i) {
    for (int j = 0; j <= mj; j++)
        if (a[i][j] == 1)
            return j;
    return -1;
}

int main() {

    int n;
    cin >> n;

    for (int ii = 0; ii < n; ii++) {
        int i, j;
        cin >> j >> i;
        j--, i--;
        mj = max(mj, j);
        mi = max(mi, i);
        a[i][j] = 1;
    }

    vector <vector <int>> dp(mi + 1, vector <int>(mj + 1, inf));

    int rf = getR(0);

    for (int j = 0; j <= mj; j++) {
        if (rf == -1)
            dp[0][j] = j;
        else {
            if (rf <= j)
                dp[0][j] = j;
            else
                dp[0][j] = rf + rf - j;
        }
    }

    for (int i = 1; i <= mi; i++) {
        int b = getR(i);
        int a = getL(i);
        for (int k = 0; k <= mj; k++) {
            for (int j = 0; j <= mj; j++) {
                if (a == -1 && b == -1)
                    dp[i][j] = min(dp[i][j], dp[i-1][k] + 1 + max(k, j) - min(k, j));
                else {
                    int l = min(k, j);
                    int r = max(k, j);
                    int res = r - l;

                    if (a < l)
                        res += 2 * (l - a);
                    if (b > r)
                        res += 2 * (b - r);
                    dp[i][j] = min(dp[i][j], dp[i-1][k] + 1 +  res);
                }
            }

        }
    }

    int ans = 1e9;
    for (int j = 0; j <= mj; j++)
        ans = min(ans, dp[mi][j]);

    cout << ans;




    return 0;
}