题目链接：区间覆盖

#include <iostream>
#include <algorithm>

using namespace std;

const int N = 100010;

int n;
struct Range
{
    int l, r;
    bool operator< (const Range &W) const
    {
        return l < W.l;
    }
}range[N];

int main()
{
    int st, ed;
    cin >> st >> ed >> n;
    for(int i = 0; i < n; i ++)
    {
        int l, r;
        cin >> l >> r;
        range[i] = {l, r};
    }
    
    sort(range, range + n);
    
    int  res = 0;
    bool success = false;
    for(int i = 0; i < n; i ++)
    {
        int j = i, r = -2e9;
        while(j < n &&  range[j].l <= st)
        {
            r = max(r, range[j].r);
            j++;
        }
        if(r < st)
        {
            res = -1;
            break;
        }
        
        res ++;
        if(r >= ed)
        {
            success = true;
            break;
        }
        
        st = r;
        i = j - 1;
    }
    
    if(!success) res = -1;
    cout << res << endl;
    
    return 0;
}