然后洛谷上RE。。。

#include <bits/stdc++.h>
using namespace std;
struct node {
    long long x, y;
};
bool cmp(node a, node b) {
    return a.x < b.x;
}
long long ans = 0;
long long n, a, b;
int main() {
    cin >> n;
    node f[n];
    for(int i = 1; i <= n; i++) {
        cin >> f[i].x >> f[i].y;
    }
    sort(f + 1, f + 1 + n, cmp);
    a = f[1].x, b = f[1].y;
    for(long long i = 2; i <= n; i++) {
        if(f[i].x > b) {
            ans += b - a, a = f[i].x, b = f[i].y;
        } else {
            b = max(f[i].y, b);
        }
    }
    cout << ans + b - a << endl;
    return 0;
}