Count ways to reach nth stairs DP

int mod = 1e9 + 7;
int countDistinctWays(int nStairs) {
    vector<int> dp(nStairs + 1, -1);
    dp[0] =1;
    dp[1] =1;
    
    for(int i=2;i<=nStairs;i++){
        dp[i] = (dp[i-1] + dp[i-2])%mod;
    }
    return dp[nStairs];
}