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>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 5e5 + 5, C = 26, MD = 1e9 + 7;
void add(int &x, int y) {
if ((x += y) >= MD) {
x -= MD;
}
if (x < 0) {
x += MD;
}
}
int n, m;
int dp[N][C], cur[2][C];
vector<int> st[2];
void pop(int i, int x) {
while (st[i].size() && st[i].back() <= x) {
for (int j = 0; j < C; ++j) {
add(cur[i][j], -dp[st[i].back()][j]);
}
st[i].pop_back();
}
}
void psh(int i) {
st[0].push_back(i);
st[1].push_back(i);
for (int j = 0; j < C; ++j) {
for (auto it : {0, 1}) {
add(cur[it][j], dp[i][j]);
}
}
}
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n >> m;
vector<array<int, 3>> events;
for (int i = 1; i <= m; ++i) {
int a, b; cin >> a >> b;
if (a < b) {
events.push_back({a + 1, b, 1});
} else {
events.push_back({b + 1, a, 0});
}
}
fill(dp[n], dp[n] + C, 1);
psh(n);
sort(events.rbegin(), events.rend());
for (int i = n - 1, j = 0; i > 0; --i) {
while (j < m && events[j][0] > i) {
pop(events[j][2], events[j][1]);
++j;
}
int sum = 1;
for (int j = 0; j < C; ++j) {
dp[i][j] = sum;
add(sum, cur[0][j]);
}
sum = 0;
for (int j = C - 1; j >= 0; --j) {
add(dp[i][j], sum);
add(sum, cur[1][j]);
}
psh(i);
}
int res = 0;
for (int i = 0; i < C; ++i) {
add(res, dp[1][i]);
}
cout << res;
return 0;
}
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