# include <iostream>
# include <cstdlib>
using namespace std;

int main()
{
    int n, a[3010], b[3010], bgx[3010], ans = 0;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i] >> b[i];
        if (i == 1) bgx[i] = a[i] + b[i]; continue;
        bgx[i] = (a[i - 1] + b[i - 1]) + ((a[i] + b[i]) - 8);
    }
    for (int i = 1; i <= n; i++)
    {
        ans += bgx[i];
    }
    cout << ans << endl;
    return 0;
}

20分，只对最后一个测试点