rt，本来是来练习差分的，结果想了半天也没想出差分，写了个暴力反而还A了。。。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;

const int N = 2e3 + 5;

int c, f1, f2, d;
int tmp[N];

int main()
{
    scanf("%d%d%d%d", &c, &f1, &f2, &d);
    for (int i = 1; i <= c; i ++ )
    {
        int a, b;
        scanf("%d%d", &a, &b);
        for (int j = a; j <= b; j ++ ) tmp[j] ++ ;
    }

    for (int i = d; i >= 1; i -- )
    {
        f1 -= tmp[i];
        if (f1 == f2)
        {
            printf("%d\n", i);
            return 0;
        }
    }
    return 0;
}