#include <bits/stdc++.h>   using namespace std;   const int N = 265000;   int n, q, a[N];   double s;   int main() {     scanf( %d%d , &n, &q);     for (int i = (0); i <= (int)(1 << n) - 1; i++) scanf( %d , &a[i]), s += a[i];     printf( %.10lf n , 1.0 * s / (1 << n));     for (int i = (1); i <= (int)q; i++) {       int x, y;       scanf( %d%d , &x, &y);       s += y - a[x];       a[x] = y;       printf( %.10lf n , 1.0 * s / (1 << n));     }   }   