#include <stdio.h>

int main(void){
    int K, N, F, i, A, T = 0, T1 = 0, T0 = 0;

    scanf("%d%d%d", &K, &N, &F);
    for (i = 0; i < F; i++) {
    	scanf("%d", &A);
    	T += A;
    }
    T1 = K * N;
    T0 = T1 - T;
    printf("%d--------------------------%d", T, T1);
    if (T <= T1) {
    	printf("%d", T0);
    }
    else {
    	printf("-1");
    }
}