#include <bits/stdc++.h>
using namespace std;
#define Task "skyscraper"
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define foru(i, l, r) for(int i = l; i <= r; i++)
#define ford(i, r, l) for(int i = r; i >= l; i--)
#define __TIME (1.0 * clock() / CLOCKS_PER_SEC)
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
typedef pair<ii, ii> iiii;
const int N = 3e4 + 5;
const int oo = 0x3f3f3f3f, mod = 1e9 + 7;
int n, m, k;
string s;
vector<int> arr;
vector<int> ed[N];
vector<pair<int, int>> ej[N];
bool find(int j, int d) {
auto it = lower_bound(ed[j].begin(), ed[j].end(), d);
return it != ed[j].end() && *it == d;
}
void process() {
static int dd[N];
int n, m, i, j, d, s, t;
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int,int>>> pq;
cin >> n >> m;
s = t = -1;
for (i = 0; i < m; i++) {
cin >> j >> d;
if (i == 0)
s = j;
else if (i == 1)
t = j;
ed[j].push_back(d);
}
for (i = 0; i < n; i++) {
sort(ed[i].begin(), ed[i].end());
ed[i].erase(unique(ed[i].begin(), ed[i].end()), ed[i].end());
}
for (i = 0; i < n; i++)
for (int d : ed[i]) {
for (j = i + d; j < n; j += d) {
ej[i].push_back({j, (j - i) / d});
if (find(j, d))
break;
}
for (j = i - d; j >= 0; j -= d) {
ej[i].push_back({j, (i - j) / d});
if (find(j, d))
break;
}
}
memset(dd, 0x3f, n * sizeof *dd);
dd[s] = 0;
pq.push({0, s});
while (!pq.empty()) {
i = pq.top().second;
pq.pop();
for (auto tmp : ej[i]) {
int j=tmp.fi, w=tmp.se;
d = dd[i] + w;
if (d < dd[j]) {
dd[j] = d;
pq.push({d, j});
}
}
}
cout << (dd[t] == oo ? -1 : dd[t]);
}
signed main() {
cin.tie(0)->sync_with_stdio(false);
if(fopen(Task".INP", "r")) {
freopen(Task".INP", "r", stdin);
freopen(Task".OUT", "w", stdout);
}
process();
cerr << "Time elapsed: " << __TIME << " s.\n";
return 0;
}
// dont stop
