#include <bits/stdc++.h> using namespace std; char x[21]; int main(){ cin >> x; if(x[0] >= 48 && x[0] <= 57){ cout << "no"; return 0; } for(int i = 1;i < strlen(x);i++){ int p = x[i]; if((p<'a' && p>'z') && (p<'A' && p>'Z') && p!='_'){ cout << "no"; return 0; } } cout << "yes"; return 0; }

#include<bits/stdc++.h>
#define int long long
#define INF 0x3f3f3f3f
#define INFSHORT 1e+3
#define INFINT 1e+10
#define INFLONGLONG 1e+19
using namespace std;
string a;
signed main(){
	cin>>a;
	for(int i=0;i<a.size();i++){
	    if(i==0) if(a[i]>=48&&a[i]<=57){
				 	 puts("no");
					 return 0;
				 }
	    if (a[i]>='a'&&a[i]<='z') continue;
		else if(a[i]>=48&&a[i]<=57) continue;
		else if(a[i]=='_') continue;
		else {
			puts("no");
			return 0;
		}
	}
	puts("yes");
}

只有90分，别抄（what）

#include <bits/stdc++.h>
using namespace std;
char x[21];
int main(){
	cin >> x;
	if(x[0] >= 48 && x[0] <= 57){
		cout << "no";
		return 0;
	}
	for(int i = 1;i < strlen(x);i++){
		int p = x[i];
		if((p<'a' && p>'z') && (p<'A' && p>'Z') && p!='_'){
			cout << "no";
			return 0;
		}
	}
	cout << "yes";
	return 0;
}