#include <iostream>
using namespace std;

int main() {
    int n, m;
    int a, b;
    while (cin >> n >> m) {
        int min_amount = n;
        for (int i = 0; i < m; ++i) {
            cin >> a >> b;
            if (a <= n)
                min_amount = min(min_amount, n - b);
        }
        cout << min_amount;
    }
}