C++ :

#include<iostream>
using namespace std;
int main(){
    long long int sum,m,n;
        cin>>n>>m;
        sum=(n+m)*(m-n+1)/2;
        cout<<sum<<endl;
      
	return 0;
}

Pascal :

var
a,b,s:longint;
d:int64; 
begin
readln(a,b);
for s:= a to b do
d:=d+s;
write(d);
end.