#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vll=vector<long long>;
template<typename T>T get(){T r;cin>>r;return r;}
template<typename T>vector<T> get_v(ll n){vector<T> v;for(int i=0;i!=n;i++){v.push_back(get<T>());};return v;}
template<typename T>void show_v(vector<T> v){
    for(ll i=0;i!=v.size()-1;i++) cout<<v[i]<<" ";cout<<*(v.end()-1)<<endl;}

int main() {
    vll a=get_v<ll>(4);
    sort(a.begin(),a.end());
    bool f=true;
    for(ll i=1;i!=a.size()&&f;i++){
        f=a[i-1]+1==a[i];
    }
    cout<<(f?"Yes":"No")<<endl;

    system("pause");
}