text: #include <bits/stdc++.h>
  
  using namespace std;
  int main(){
      ios_base::sync_with_stdio(false);
      cin.tie(NULL);
      int x,y,z,n1,n2;
      while(cin>>x>>y>>z){
          if(z==0) {
              cout<<"0\n";
              continue;
          }
          if((x-y)==0){
              if(z==x) cout<<"1\n";
              else cout<<"-1\n";
              continue;
          }
          if(z%(x-y)==0){
              n1 = z/(x-y) + 1;
              n1 = 2*(n1-1);
          }
          else
              n1 = 100000000;
          if((z-x)%(x-y)==0){
              n2 = (z-x)/(x-y) + 1;
              n2 = 2*n2-1;
          }
          else 
              n2 = 100000000;
          if(n1<0) n1=100000000;
          if(n2<0) n2=100000000;
          if(n1<n2) cout<<n1;
          else if(n2<n1) cout<<n2;
          else cout<<"-1";
          cout<<"\n";
      }
      return 0;
  }