#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int inf = 1e9;
const int N = 6000050;struct edge
{int to , w , nxt;
} e[15000005];int n , m , block , tot , S , T;
int head[N] , dist[N];
bool inq[N];template <typename T> inline void chkmax(T &x,T y) { x = max(x,y); }
template <typename T> inline void chkmin(T &x,T y) { x = min(x,y); }
template <typename T> inline void read(T &x)
{T f = 1; x = 0;char c = getchar();for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + c - '0';x *= f;
}
inline int id(int x , int y)
{return y * n + x;
}
inline void addedge(int u , int v , int w)
{++tot;e[tot] = (edge){v , w , head[u]};head[u] = tot;
}
inline int SPFA()
{queue< int > q;q.push(S);memset(dist , 0x3f , sizeof(dist));dist[S] = 0;inq[S] = true;while (!q.empty()){int cur = q.front();q.pop();inq[cur] = false;for (int i = head[cur]; i; i = e[i].nxt){int v = e[i].to , w = e[i].w;if (dist[cur] + w < dist[v]){dist[v] = dist[cur] + w;if (!inq[v]){inq[v] = true;q.push(v);        }        }        }                }return dist[T] != 0x3f3f3f3f ? dist[T] : -1;
}int main()
{read(n); read(m);block = min((int)sqrt(n) , 100);for (int i = 1; i <= block; ++i){for (int j = i; j < n; ++j){addedge(id(j , i) , id(j - i , i) , 1);addedge(id(j - i , i) , id(j , i) , 1);        }    for (int j = 0; j < n; ++j) addedge(id(j , i) , id(j , 0) , 0);}for (int k = 1; k <= m; ++k){int Bi , Pi;read(Bi); read(Pi);if (Pi <= block) addedge(id(Bi , 0) , id(Bi , Pi) , 0);    else{for (int i = Bi + Pi; i < n; i += Pi) addedge(id(Bi , 0) , id(i , 0) , (i - Bi) / Pi);for (int i = Bi - Pi; i >= 0; i -= Pi) addedge(id(Bi , 0) , id(i , 0) , (Bi - i) / Pi);}if (k == 1) S = id(Bi , 0);if (k == 2) T = id(Bi , 0);}printf("%d\n" , SPFA());return 0;}