全WA了！！

#include <bits/stdc++.h>

using namespace std;

int main()
{
    short l, m, tree = 0;
    bool vis[10010] = {0};
    
    cin >> l >> m;
    
    for (int a = 0; a <= l; a++)
    {
    	vis[a] = 0;
	}
	
	for (int b = 0; b < l; b++)
	{
		int i, j;
		
		cin >> i >> j;
		
		for (int c = i; c <= j; c++)
		{
			if (vis[c] = 0)
			{
				vis[c] = 1;
			}
		}
	}
	
	for(int d = 0; d <= l; d++)
    {
        if(vis[d] == 0)
        {
        	tree++;
		}
    }
    
    cout << tree;
    
    return 0;
}

为什么本机测试没有答案呢？