WA求助

#include<iostream>
using namespace std;
int main(){
    int a,b,c,d,e,f;
    cin>>a>>b>>c;
    d = b-a;
    e = a;
    for(int i = 1;i<c;i++){
        e += d;
    }
    f = a+e;
    cout<<f*c/2;
    return 0;
}