#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 30000
#define M_ 11111111
int min(int a, int b) { return a < b ? a : b; }
void append(int **eh, int *eo, int i, int h) {
int o = eo[i]++;
if (o >= 2 && (o & o - 1) == 0)
eh[i] = (int *) realloc(eh[i], o * 2 * sizeof *eh[i]);
eh[i][o] = h;
}
int main() {
static int *eh1[N + 1], eo1[N + 1], *eh2[N + 1], eo2[N + 1], ii[M_], hh[N], pp[M_], qq[M_], dd[M_], qu[M_ * 2];
int n, m, m_, h, i, j, o, head, cnt;
scanf("%d%d", &n, &m);
for (i = 0; i <= n; i++) {
eh1[i] = (int *) malloc(2 * sizeof *eh1[i]);
eh2[i] = (int *) malloc(2 * sizeof *eh2[i]);
}
for (h = 0; h < m; h++) {
scanf("%d%d", &i, &j), j = min(j, n);
ii[h] = i;
append(eh1, eo1, i, h);
append(eh2, eo2, j, h);
}
if (ii[0] == ii[1]) {
printf("0\n");
return 0;
}
m_ = m;
memset(hh, -1, n * sizeof *hh);
memset(pp, -1, sizeof pp), memset(qq, -1, sizeof qq);
for (j = 1; j < n; j++) {
for (o = eo2[j]; o--; ) {
h = eh2[j][o];
hh[ii[h]] = h;
}
for (o = eo2[j]; o--; ) {
int h_, h1;
h = eh2[j][o];
if (hh[ii[h]] == -1)
continue;
h_ = -1;
for (i = ii[h] % j; i < n; i += j) {
if (hh[i] == -1)
ii[h1 = m_++] = i;
else
h1 = hh[i], hh[i] = -1;
if (h_ != -1)
qq[h_] = h1, pp[h1] = h_;
h_ = h1;
}
}
}
for (h = 0; h < m_; h++)
dd[h] = m_;
head = m_, cnt = 0;
dd[0] = 0, qu[head + cnt++] = 0;
while (cnt) {
int d, o, h_;
h = qu[cnt--, head++], i = ii[h], d = dd[h];
if (h == 1) {
printf("%d\n", d);
return 0;
}
if (eo1[i]) {
for (o = eo1[i]; o--; ) {
h_ = eh1[i][o];
if (dd[h_] > d)
dd[h_] = d, qu[cnt++, --head] = h_;
}
eo1[i] = 0;
}
d++;
h_ = pp[h];
if (h_ != -1 && dd[h_] > d)
dd[h_] = d, qu[head + cnt++] = h_;
h_ = qq[h];
if (h_ != -1 && dd[h_] > d)
dd[h_] = d, qu[head + cnt++] = h_;
}
printf("-1\n");
return 0;
}
