#include<bits/stdc++.h>
using namespace std;
#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define REP(i, n) FOR(i, 0, n)
#define _ << " " <<
#define sz(x) ((int) x.size())
#define pb(x) push_back(x)
#define TRACE(x) cerr << #x << " = " << x << endl
typedef long long ll;
typedef pair<int, int> point;
const int mod = 1e9 + 7;
int add(int x, int y) {x += y; if(x >= mod) return x - mod; return x;}
int sub(int x, int y) {x -= y; if(x < 0) return x + mod; return x;}
int mul(int x, int y) {return (ll) x * y % mod;}
const int MAXN = 3e4 + 5;
int n, m, d[MAXN], dist[MAXN], start[MAXN];
vector <point> E[MAXN];
set<int> S[MAXN];
void update(int pos, int range){
int cnt = 1;
for(int curr = pos + range; curr < n; curr += range){
if(S[curr].find(range) != S[curr].end()) break;
E[pos].pb(point(curr, cnt));
cnt ++;
}
cnt = 1;
for(int curr = pos - range; curr >= 0; curr -= range){
if(S[curr].find(range) != S[curr].end()) break;
E[pos].pb(point(curr, cnt));
cnt ++;
}
}
int dijkstra(){
memset(d, -1, sizeof(d));
priority_queue<point> Q;
Q.push(point(0, start[0]));
while(!Q.empty()){
while(!Q.empty() && d[Q.top().second] != -1) Q.pop();
if(Q.empty()) break;
auto tmp = Q.top(); Q.pop();
int distance = -tmp.first;
int cvor = tmp.second;
d[cvor] = distance;
for(auto edge : E[cvor]){
int ncvor = edge.first, ndist = edge.second;
if(d[ncvor] != -1) continue;
Q.push(point( -(distance + ndist), ncvor ));
}
}
return d[start[1]];
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n >> m;
REP(i, m){
cin >> start[i] >> dist[i];
if(i != 1)
S[start[i]].insert(dist[i]);
}
REP(i, m)
if(i != 1)
update(start[i], dist[i]);
cout << dijkstra();
}
