#include <algorithm>
#include <array>
#include <cassert>
#include <iomanip>
#include <iostream>
#include <map>
#include <vector>

using namespace std;

void solve() {
  int n, m;
  cin >> n >> m;
  int bad = 0;
  int ret = n - 1;
  while(m--) {
    int a, b;
    cin >> a >> b;
    if(a > b) bad++;
    else if(a + 1 == b) ret--;
  }
  if(bad) ret = -1;
  cout << ret << "\n";
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int t;
  cin >> t;
  while(t--) solve();
}