#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; } template inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); } template inline T sqr(T x) { return x*x; } typedef pair P; typedef long long ll; typedef unsigned long long ull; #define For(i,a,b) for(int (i) = (a);i < (b);(i)++) #define rep(i,n) For(i,0,n) #define clr(a) memset((a), 0 ,sizeof(a)) #define mclr(a) memset((a), -1 ,sizeof(a)) #define all(a) (a).begin(),(a).end() #define rall(a) (a).rbegin(), (a).rend() #define sz(a) (sizeof(a)) #define Fill(a,v) fill((int*)a,(int*)(a+(sz(a)/sz(*(a)))),v) bool cheak(int x, int y, int xMax, int yMax){ return x >= 0 && y >= 0 && xMax > x && yMax > y; } const int dx[4] = { -1, 0, 1, 0 }, dy[4] = { 0, 1, 0, -1 }; const int mod = 1000000007; const int INF = 2147483647; int main() { int k, n, f; cin >> k >> n >> f; int sum = 0; rep(i, f){ int a; cin >> a; sum += a; } int ans = k*n - sum; if (ans < 0) ans = -1; cout << ans << endl; return 0; }