Codeforces 452C Magic Trick (概率)

#include <iostream>#include <string>#include <cstring>#include <cstdio>#include <algorithm>#include <cmath>#include <vector>#include <deque>#include <map>#include <bitset>#include <queue>#pragma comment(linker, "/STACK:102400000,102400000")using namespace std;#define LL long long#define ULL unsigned long long#define eps 1e-9#define N 100020#define M 100010#define pii pair<int,int>#define MP make_pair#define inf 0x3f3f3f3f#define md (ll+rr>>1)#define lson ll, md, ls#define rson md + 1, rr, rs#define ls (i << 1)#define rs (ls | 1)#define mod 1000000007#define Pi acos(-1.0)int main() {int n, m;scanf("%d%d", &n, &m);if(n * m == 1) {puts("1");return 0;}long double ans = (n - 1.0) / n;ans *= (m - 1);ans /= (n * m - 1);ans += 1.0 / n;printf("%.12lf\n", (double)ans);return 0;}