#include <bits/stdc++.h>   using namespace std;   const int maxn = 2e5 + 5;   int n;   long long r, a[(1LL << 20)], x, y;   int main() {     scanf( %d %I64d , &n, &r);     long long sum = 0;     double qpw = 1;     for (int i = 1; i <= n; ++i) qpw *= 2;     for (int i = 1; i <= qpw; ++i) {       scanf( %I64d , &a[i]);       sum += a[i];     }     printf( %.6f n , sum * 1.0 / qpw);     double ans = 0;     while (r--) {       scanf( %I64d %I64d , &x, &y);       sum += (y - a[x + 1]);       a[x + 1] = y;       ans = sum * 1.0 / qpw;       printf( %.6f n , ans);     }     return 0;   }   