#include<iostream>
#include<algorithm>

int main(){
    int a,b,c,d;
    std::cin >>a>>b>>c>>d;
    auto ans = (b-a)*(d-c);
    
    auto left = std::min(b,d);
    auto right = std::max(a,c);
    
    ans -= left<right ? (right-left+1):0;
    std::cout << ans<<"\n";
    
    return 0;
}