progressive sum

/**
 * Auto-generated code below aims at helping you parse
 * the standard input according to the problem statement.
 **/

var inputs = readline().split(' ');
 M = parseInt(inputs[0]);
 D = parseInt(inputs[1]);
 N = parseInt(inputs[2]);
t = []
while(N>0)
{
    t.push(M);
    M = D+M;
    N--;

}

console.log(t.reduce((a,b)=>a+b));
//