#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

int main(){
    vector<int> v(4);
    cin >> v[0] >> v[1] >> v[2] >> v[3];
    if(
        v[0]+1 == v[1] &&
        v[1]+1 == v[2] &&
        v[3]+1 == v[3]
    ) cout << "Yes" << endl;
    else cout << "No" << endl;
    return 0;
}