#include <bits/stdc++.h>
using namespace std;
#define MAXN 30005
#define pb push_back
#define mp make_pair
typedef pair<int, int> pii;
int N, M, S, E;
int D[MAXN][200];
bool V[MAXN][200];
vector <int> doge[MAXN];
int main()
{
scanf("%d%d", &N, &M);
for (int i=0;i<M;i++){
int b, p;
scanf("%d%d", &b, &p); doge[b].pb(p);
V[b][p] = 1;
if (!i) S = b;
if (i == 1) E = b;
}
for (int i=0;i<N;i++) for (int j=0;j<200;j++) D[i][j] = 2e9;
priority_queue <pii> que;
D[S][0] = 0;
que.push(mp(0, S*200));
while (!que.empty()){
int q = que.top().second, d = -que.top().first; que.pop();
int jmp = q%200; q /= 200;
if (D[q][jmp] != d) continue;
if (jmp){
if (D[q][0] > d)
D[q][0] = d, que.push(mp(-D[q][0], q*200));
if (q >= jmp && D[q-jmp][jmp] > d+1)
D[q-jmp][jmp] = d+1, que.push(mp(-d-1, (q-jmp)*200+jmp));
if (q+jmp < N && D[q+jmp][jmp] > d+1)
D[q+jmp][jmp] = d+1, que.push(mp(-d-1, (q+jmp)*200+jmp));
continue;
}
for (int p: doge[q]){
if (p > 199){
for (int i=q-p;i>=0;i-=p)
if (D[i][0] > d+(q-i)/p)
D[i][0] = d+(q-i)/p, que.push(mp(-D[i][0], i*200));
for (int i=q+p;i<N;i+=p)
if (D[i][0] > d+(i-q)/p)
D[i][0] = d+(i-q)/p, que.push(mp(-D[i][0], i*200));
}else{
if (D[q][p] > d)
D[q][p] = d, que.push(mp(-d, q*200+p));
}
}
}
printf("%d\n", D[E][0] < 2e9 ? D[E][0] : -1);
}
