#include <bits/stdc++.h>
using namespace std;
#define N 30000
#define INF 0x3f3f3f3f
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;
}
int main() {
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;
scanf("%d%d", &n, &m);
s = t = -1;
for (i = 0; i < m; i++) {
scanf("%d%d", &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 [j, w] : ej[i]) {
d = dd[i] + w;
if (d < dd[j]) {
dd[j] = d;
pq.push({d, j});
}
}
}
printf("%d\n", dd[t] == INF ? -1 : dd[t]);
}
