30分求助哪里有问题

#include<iostream>
using namespace std;

int main()
{
    int l, m, u, v, tree = 0;
    cin >> l >> m;
    int trees[10000] = {};
    for (int i = 0; i <= l; i++)
        trees[i] = 1;
    for (int i = 1; i <= m; i++)
    {
        cin >> u >> v;
        for (; u <= v; u++)
            trees[u] = 0;
    }
    for (int i = 0; i < 10000; i++)
        tree += trees[i];
    cout << tree << endl;
}