This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<bits/stdc++.h>
#define ll int
#define pll pair<ll, ll>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define ld long double
using namespace std;
const long long maxn=500005, Mod=1e9+7;
vector <pll> e[maxn];
long long dp[maxn][26];
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
ll n, m; cin >> n >> m;
for (ll i=1; i<=m; i++)
{
ll l, r; cin >> l >> r;
if (l>r)
{
swap(l, r);
e[l+1].pb({0, l});
e[r+1].pb({0, -l});
}
else
{
e[l+1].pb({1, l});
e[r+1].pb({1, -l});
}
}
multiset <ll> S[2];
for (ll i=0; i<26; i++) dp[1][i]=i+1;
for (pll i:e[1]) S[i.fi].insert(i.se);
for (ll i=2; i<=n; i++)
{
for (pll i:e[i])
{
if (i.se<0) S[i.fi].erase(S[i.fi].find(-i.se));
else S[i.fi].insert(i.se);
}
ll p0=(S[0].size() ? *prev(S[0].end()) : 0), p1=(S[1].size() ? *prev(S[1].end()) : 0);
for (ll d=0; d<26; d++)
{
if (d) dp[i][d]=(dp[i][d]+dp[i-1][d-1]-dp[p0][d-1]+Mod)%Mod;
dp[i][d]=(dp[i][d]+(dp[i-1][25]-dp[p1][25]-dp[i-1][d]+dp[p1][d])+Mod*2)%Mod;
if (d) dp[i][d]=(dp[i][d]+dp[i][d-1])%Mod;
}
for (ll d=0; d<26; d++) dp[i][d]=(dp[i][d]+dp[i-1][d])%Mod;
}
cout << dp[n][25];
}
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