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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3. #define MOD 1000000007
  4. #define LL long long
  5. #define out(x) cout << #x << " : " << x << "\n";
  6.  
  7. int firstKdigit(int p, int k) {
  8.    double lg = p * log10(2) / log10(10);
  9.    double f = lg - floor(lg) + k - 1;
  10.    return pow(10, f);  
  11. }
  12.  
  13. int main() {
  14.    int n; cin >> n;
  15.    int k = log10(n) + 1;
  16.    bool ok = false;
  17.    for(int i = 0;i <= 100000000;i++) {
  18.       int fk = firstKdigit(i, k);
  19.       //cout << fk << " ";
  20.       if(fk == n) {
  21.          cout << i << "\n";
  22.          ok = true;
  23.          break;
  24.       }
  25.    }
  26.    if(!ok) cout << -1;
  27.    return 0;               
  28. }
  29.  
  30.  
Success #stdin #stdout 0s 3100KB
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https://ideone.com/1vviKq
language:
C++ (gcc 8.3)
created:
8 years ago
visibility:
public

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