#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5, MOD = 1e9 + 7;
const long long L = 1e18;
#define ll long long
#define fi first
#define se second
#define yes {cout << "YES";return;}
#define no {cout << "NO"; return;}
#define pb push_back
#define MASK(i) (1ll << (i))
#define BIT(i, s) ((1ll << (i)) & (s))
#define all(a) a.begin(), a.end()
int n, m;
int a[N], b[N];
bool visited[N], Left[N], Right[N];
int dist[N];
vector <int> s[N];
priority_queue <pair <int, int>, vector<pair <int, int>>, greater <pair <int, int>>> p;
void solve()
{
cin >> n >> m;
for (int i = 0; i < m; i++){
cin >> a[i] >> b[i];
s[a[i]].pb(i);
}
memset(dist, 0x3f, sizeof dist);
for (int x: s[a[0]]){
dist[x] = 0;
p.push({0, x});
}
while(p.size()){
int i = p.top().fi;
int j = p.top().se;
p.pop();
if (visited[j]) continue;
visited[j] = true;
bool ok = true;
if (!Left[j]){
for (int l = a[j] - b[j]; ok && l >= 0; l -= b[j]){
for (int x: s[l]){
if (visited[x]) break;
int y = (a[j] - l) / b[j];
dist[x] = min(dist[x], i + y);
if (b[x] == b[j]){
if (ok) p.push({dist[x], x});
Right[x] = true;
ok = false;
} else p.push({dist[x], x});
}
}
}
ok = true;
if (!Right[j]){
for (int l = a[j] + b[j]; ok && l < n; l += b[j]){
for (int x: s[l]){
if (visited[x]) break;
int y = (l - a[j]) / b[j];
dist[x] = min(dist[x], i + y);
if (b[x] == b[j]){
if (ok) p.push({dist[x], x});
Left[x] = true;
ok = false;
} else p.push({dist[x], x});
}
}
}
}
cout << (dist[1] == dist[m + 1] ? -1 : dist[1]);
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);cout.tie(NULL);
if (fopen("VKhang.inp","r")){
freopen("VKhang.inp","r",stdin);
freopen("VKhang.out","w",stdout);
}
solve();
return 0;
}
컴파일 시 표준 에러 (stderr) 메시지
skyscraper.cpp: In function 'int main()':
skyscraper.cpp:75:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
75 | freopen("VKhang.inp","r",stdin);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
skyscraper.cpp:76:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
76 | freopen("VKhang.out","w",stdout);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
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