func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long inf = INT_MAX, df = 3e5 + 7;   long long i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, dp[df], pos[df],       a[df];   inline long long read() {     long long x = 0, y = 1;     char ch = getchar();     while (ch >  9  || ch <  0 ) y = (ch ==  - ) ? -1 : 1, ch = getchar();     while (ch >=  0  && ch <=  9 )       x = (x << 3) + (x << 1) + ch -  0 , ch = getchar();     return x * y;   }   bool chk(long long x) {     dp[1] = 0;     for (long long i = (2); i <= (n); ++i) {       dp[i] = dp[i - 1];       if (dp[i - 1] >= pos[i] - 1) dp[i] = max(dp[i], pos[i] + x);       if (dp[i - 1] >= pos[i] - x - 1) dp[i] = max(dp[i], pos[i]);       if (dp[i - 2] >= pos[i] - x - 1 && i > 2)         dp[i] = max(dp[i], pos[i - 1] + x);     }     if (dp[n] >= m - x - 1) return 1;     dp[2] = max(pos[2], x);     for (long long i = (3); i <= (n); ++i) {       dp[i] = dp[i - 1];       if (dp[i - 1] >= pos[i] - 1) dp[i] = max(dp[i], pos[i] + x);       if (dp[i - 1] >= pos[i] - x - 1) dp[i] = max(dp[i], pos[i]);       if (dp[i - 2] >= pos[i] - x - 1) dp[i] = max(dp[i], pos[i - 1] + x);     }     if (dp[n] >= min(m - 1, m + pos[2] - x - 1)) return 1;     return 0;   }   int main() {     m = read();     n = read();     long long now = 1;     for (long long i = (1); i <= (n); ++i) a[i] = read(), a[i + n] = a[i] + m;     if (n == 1) {       return printf( %lld n , m - 1), 0;     }     for (long long i = (2); i <= (n); ++i)       if (a[now + 1] - a[now] < a[i + 1] - a[i]) now = i;     long long le = 0, rig = a[now + 1] - a[now] - 1, mid = 0, ans = 0;     for (long long i = (1); i <= (n); ++i) pos[i] = a[i + now];     long long cur = pos[1];     for (long long i = (1); i <= (n); ++i) pos[i] -= cur;     while (le <= rig) {       mid = (le + rig) >> 1;       if (chk(mid))         ans = mid, rig = mid - 1;       else         le = mid + 1;     }     return printf( %lld n , ans), 0;   }   