#include <stdio.h>

typedef struct {
	int key, id;
} data;

void merge_sort(int n, data x[])
{
	static data y[100001] = {};
	if (n <= 1) return;
	merge_sort(n / 2, &(x[0]));
	merge_sort((n + 1) / 2, &(x[n/2]));
	
	int i, p, q;
	for (i = 0, p = 0, q = n / 2; i < n; i++) {
		if (p >= n / 2) y[i] = x[q++];
		else if (q >= n) y[i] = x[p++];
		else y[i] = (x[p].key < x[q].key)? x[p++]: x[q++];
	}
	for (i = 0; i < n; i++) x[i] = y[i];
}

int main()
{
	int i, N, M, B[100001], C[100001];
	data d[100001];
	scanf("%d", &N);
	scanf("%d", &M);
	for (i = 0; i < M; i++) {
		scanf("%d %d", &(B[i]), &(C[i]));
		d[i].key = B[i];
		d[i].id = i;
		C[i] = (C[i] % B[i] + B[i]) % B[i];
	}
	merge_sort(M, d);
	
	int j;
	for (i = C[d[M-1].id]; i <= N; i += B[d[M-1].id]) {
		for (j = M - 2; j >= 0; j--) if (i % B[d[j].id] != C[d[j].id]) break;
		if (j < 0) break;
	}
	if (i <= N) printf("%d\n", i);
	else printf("NaN\n");
	fflush(stdout);
	return 0;
}