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#include <iostream> #include <cstdio> #include <cmath> #include <vector> using namespace std; int n; int arr[1300000]; vector<int> v; int main() { int lo,hi,mid; for(int i=0;i<1300000;i++) { arr[i]=0; } for(int i=2;i<sqrt(1300000);i++) { if(arr[i]==0) { for(int j=i+i;j<1300000;j+=i) { arr[j]=1; } } } for(int i=2;i<1300000;i++) { if(arr[i]==0) { v.push_back(i); } } //cout<<v.size()<<"\n"; while(scanf("%d",&n),n) { if(arr[n]==0) { cout<<0<<"\n"; continue; } lo=0;hi=v.size()-1; while(hi-lo>1) { if((hi-lo)%2) { mid=lo+(hi-lo)/2+1; } else mid=lo+(hi-lo)/2; //cout<<lo<<" "<<mid<<" "<<hi<<" "<<n<<"\n"; //getchar(); if(n<v[mid]) { hi=mid; } else { lo=mid; } } cout<<v[hi]-v[lo]<<"\n"; } return 0; } |