WA on Test#3~7，求调

#include <bits/stdc++.h>
using namespace std;

int n, a[20010][2], f[20010][2];

signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	
	cin >> n;
	for (int i = 1; i <= n; i++)
		cin >> a[i][0] >> a[i][1];
	f[1][0] = (a[1][1] << 1) - a[1][0] - 1;
	f[1][1] = a[1][1] - 1;
	
	for (int i = 2; i <= n; i++)
		for (int j = 0; j <= 1; j++)
			f[i][j] = min(f[i - 1][0] + a[i][1] - a[i][0] + abs(a[i][!j] - a[i - 1][0]),
				f[i - 1][1] + a[i][1] - a[i][0] + abs(a[i][j] - a[i - 1][1])) + 1;
	cout << min(f[n][1] - a[n][1] + n, f[n][0] - a[n][0] + n);
}