#include <stdio.h>
int main(void)
{
    int K = 0, N = 0, F = 0, A = 0;
    int TotalAge = 0;
    int Totalmame = 0;
    int i = 0;

    scanf("%d%d%d", &K, &N, &F);

    for (i = 0; i < F; i++){
        scanf("%d", &A);
        TotalAge += A;
    }
    Totalmame = K * N;
    if (Totalmame - TotalAge >= 0){
        printf("%d", Totalmame - TotalAge);
    }else{
        printf("%d", -1);
    }

    return 0;
}