b966

/* b966             */
/* AC (0.4s, 412KB) */
#include <cstdio>
#include <vector>
#include <algorithm>

using namespace std;

typedef struct _line
{
        int s, e;
}line;

bool cmp(line a, line b)
{
        if(a.s == b.s) return a.e < b.e;
        return a.s < b.s;
}

vector<line> arr;

int main()
{
       for(int N; ~scanf("%d", &N) && N;)
       {
             arr.resize(N);
             int len = 0;
             for(int i = 0, s, e; i < N && scanf("%d %d", &s, &e); arr[i].s = s, arr[i].e = e, ++i);
             sort(arr.begin(), arr.end(), cmp);
             for(int i = 0; i < N; ++i)
             {
                   int s = arr[i].s, e = arr[i].e;
                   while(i + 1 < N && e > arr[i + 1].s)
                   {
                            if(e < arr[i + 1].e)
                                e = arr[i + 1].e;
                            ++i;
                   }
                   len += e - s;
             }
             printf("%d\n", len);
       }
       return 0;
}