#include <stdio.h>
#include <stdlib.h>

using namespace std;

int nr0(int n){
    int nr=0;
    int p=5;
    while(p<=n){
        nr+=n/p;
        p*=5;
    }
    return nr;
}

int main()
{
    FILE *fin, *fout;
    int n,p5,p,st,dr,mij,zero;
    fin=fopen("factorial.in","r");
    fout=fopen("factorial.out","w");
    fscanf(fin,"%d",&p);
    st=1;
    dr=100000000;
    while(st<dr){
        mij=(st+dr)/2;
        zero=nr0(mij);
        if(zero>=p)
            dr=mij;
        else
            st=mij+1;
    }
    fprintf(fout,"%d",st);
    fclose(fin);
    fclose(fout);
    return 0;
}